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공학박사학위논문

Synthesis and characterization of phthalocyanine dyes for liquid crystal display black matrix and phenoxazine dyes for dye-sensitized solar cells

액정 디스플레이 블랙 매트릭스 용 프탈로시아닌 염료 및

염료감응 태양전지용 페녹사진 염료의

합성과 특성에 대한 연구

2014년 2월

서울대학교 대학원

재료공학부

이 우 성

i

Abstract

Synthesis and characterization of phthalocyanine dyes for liquid crystal display black matrix and phenoxazine

dyes for dye-sensitized solar cells

Lee Woosung

Department of Material Science and Engineering

The Graduated School

Seoul National University

The most frequently used material for black matrix is carbon black, which has

advantages of high thermal stability and high light absorption. However, black

matrices fabricated with carbon black have high dielectric constants, causing

electrical signal transduction errors on thin film transistors. To avoid this

problem, organic pigment BMs with a low dielectric constant can be used, but

their low spectral properties due to the lower molar extinction coefficient of

ii

organic pigments have limited their applications.

In general, dyes have much lower dielectric constants compared with carbon

black, and show much higher light absorption properties than organic pigments.

Thus, if dyes are used for the manufacture of the black matrix, the high

dielectric constant and low light absorption of conventional black matrices can

be overcome. On the other hand, dyes generally have low thermal stability

compared to carbon black and organic pigments. Therefore, the dyes for black

matrix need to be structurally stable. They also should have good solubility in

industrial solvents, such as propylene glycol methyl ether acetate and

cyclohexanone. In addition, as dyes generally have sharp absorption ranges,

dye-based black matrices need to be fabricated by mixing red, green and blue

dyes, or cyan, magenta and yellow dyes, as well as by mixing dyes and carbon

black..

In this study, green metal-free phthalocyanine dyes with high thermal stability

and high solubility were designed. Three phthalocyanine dyes were synthesized

by introducing substituents including alkyl or alkoxy groups to the peripheral

position of the phthalocyanine rings, and their spectral properties, solubilities

and thermal stabilities were measured. In addition, dye-based black matrices

were fabricated and their optical and dielectric properties were examined.

iii

The metal-free phthalocyanine dyes showed the increase in solubility due to

bulky functional substituents at the peripheral positions of them. In addition, the

dyes including terminal alkoxy groups showed suitable thermal stability for

commercial use due to terminal alkoxy groups are stable at postbaking

temperature. Since all dyes had high molar extinction coefficients, dye-based

black matrices absorbed light in the visible region with the small amounts of the

dyes. The dielectric constants of the black matrices containing more than 30wt%

of dyes were significantly lower than that of the black matrix prepared with

carbon black only.

Furthermore, greenish zinc phthalocyanine dyes and reddish perylene dyes

were synthesized and employed to fabricate black matrices with low dielectric

constant and light absorption in the whole visible region. The spectral and

thermal properties and solubility of the prepared dyes were investigated, and

optical, thermal and dielectric properties of the dye-based black matrices were

examined. For further investigation of surface morphology of the black matrix

films, dye-based black matrices were probed by field emission scanning

electron microscopy and atomic force microscopy.

The dye-based black matrix showed the high thermal stability due to the rigid

molecular structures of the dyes. In addition, due to the low dielectric

iv

characteristics of the dye, the dielectric constants of the dye-based black

matrices were significantly lower than that of the black matrix prepared with

carbon black only. However, the low solubility of the dyes in industrial solvents

and dye aggregations in the baking process limited the input of the dye in the

black matrix resist, resulting in low light absorption of the dye-based black

matrix.

Dye-sensitized solar cells have attracted considerable attention as promising

solar devices and the Ru complex dyes typical used as sensitizers in dye-

sensitized solar cells have shown high electronic conversion efficiencies of over

11% with good stability. However, high production cost and difficulties in

purification of Ru complex dyes have limited their development for large-scale

applications. Recently, more attention has been paid to sensitizers without Ru

(metal-free organic dyes and organometallic dyes) due to their lower cost,

easier modification and purification, high molar extinction coefficient, and

environmental friendliness.

Phenoxazine-based sensitizers have exhibited higher conversion efficiencies

than triphenylamine and phenothiazine-based sensitizers, which are structurally

similar. This is because phenoxazine-based sensitizers, with electron-rich

nitrogen and oxygen heteroatoms, have stronger electron-donating ability than

v

triphenylamine and phenothiazine-based sensitizers. phenoxazine-based

sensitizers also show sufficient electrochemical properties for use in dye-

sensitized solar cells. However, despite their potential for application to dye-

sensitized solar cells, phenoxazine-based sensitizers have not been studied

extensively.

In this research, to study effects of conjugated bridges with a phenoxazine

moiety on photovoltaic performance, five-membered heterocyclic rings were

introduced as a conjugated bridge unit to phenoxazine molecules. Furthermore,

to improve the donating power and molar extinction coefficient, an ethoxy

phenyl ring was substituted in the 7 position of the phenoxazine -furan dye as

an additional donor. Based on these strategies, three organic dyes were

synthesized and the photophysical, electrochemical, and photovoltaic properties

of the solar cells based on these dyes were investigated.

The introduced heterocyclic bridge units furan and thiophene improved the

short-circuit current due to the red-shifted absorption spectra of the dyes. The

ethoxyphenyl ring introduced to the phenoxazine moiety as an additional donor

broadened the spectrum of the dye, while the reduced adsorption of the dye

caused by its non-planar structure limited the enhancement of the short-circuit

vi

current. As a result, among the synthesized dyes, the one with furan as a bridge

unit showed the best overall conversion efficiency of 5.26%.

In addition, to study the effects of the number of anchoring groups and N-

substitution on the performance of phenoxazine dyes in dye-sensitized solar

cells, cyanoacrylic acid as an additional anchoring group was introduced to the

phenoxazine for efficient electron extraction from the donor part, and an N-

methoxyphenyl unit was added to suppress dye aggregation. Based on these

strategies, four phenoxazine derivatives were synthesized and the photophysical,

electrochemical and photovoltaic properties of the solar cells based on these

dyes were investigated.

The additional cyanoacrylic acid acceptor improved the short-circuit current

because it widened the absorption ranges of the dyes, although it also increased

the recombination rate. The N-methoxyphenyl unit decrease charge

recombinations, resulting in higher open-circuit voltage. However, the bulky

substituent decreased the amount of dye absorbed on the TiO2. As a result, the

fabricated cells with the four dyes exhibited similar overall conversion

efficiencies and, of these cells, the solar cell based on the N-4-methoxyphenyl

mono-cyanoacrylate substituted dye showed the highest conversion efficiency

of 5.09%.

vii

KEYWORDS: Liquid crystal display, Black Matrix, dyes, Phthalocyanine,

Dielectric constant, Light absorption, Solubility, Thermal stability, Light

Absorption, Dye-sensitized solar cells, Phenoxazine dyes, Five-membered

heterocyclic bridges, Ethoxyphenyl substitution, Dihedral angle, Dye

adsorption, Di-anchor, N-substituent, Dihedral angle, Dye adsorption

Student Number: 2007-20735

viii

Contents

Abstract

Contents

List of Tables

List of Schemes

List of Figures

Chapter 1. Introduction

1.1 An overview of LCDs (Liquid Crystal Displays)

1.2 Structures and requirements of LCD black matrix

1.3 Fabrication of black matrix pattern using black matrix photo-resist

1.4 An overview of DSSCs (Dye-sensitized Solar Cells)

1.5 Operating principle and fabrication of DSSCs

1.6 Requirements of organic sensitizers

1.7 References

ix

Chapter 2. Synthesis and characterization of solubility enhanced

metal-free phthalocyanines for liquid crystal display black

matrix of low dielectric constant

2.1 Introduction

2.2 Experimental

2.2.1 General

2.2.2 Synthesis

2.2.3 Preparation of dye-based black matrix

2.2.4 Measurement of spectral and chromatic properties

2.2.5 Measurement of solubility

2.2.6 Measurement of thermal stability

2.2.7 Geometry optimization of the synthesized dyes

2.3 Results and Discussion

2.3.1 Synthesis of dyes

2.3.2 Solubility

2.3.3 UV-vis absorption spectra

x

2.3.4 Thermal properties

2.3.5 Dielectric properties

2.4 Conclusions

2.5 References

Chapter 3. Analysis and characterization of dye-based black

matrix film of low dielectric constant containing phthalocyanine

and perylene dyes

3.1 Introduction

3.2 Experimetal

3.2.1 General

3.2.2 Synthesis


3.2.4. Measurement of spectral and optical properties

3.2.5 Investigation of solubility

xi


3.2.7 Field emission scanning electron microscopy and Atomic force

microscopy


3.3.1 Properties of dyes

3.3.2 Spectral and optical properties of dye-based black matrix

3.3.3 Thermal properties of dye-based black matrix

3.3.4 Dielectric properties of dye-based black matrix

3.3.5 Surface investigation of BM film by FE-SEM and AFM

3.4 Conclusion

3.5 References

Chapter 4. The effect of five-membered heterocyclic bridges and

ethoxyphenyl substitution on the performance of phenoxazine-

based dye-sensitized solar cells

xii

4.1 Introduction

4.2 Experimental

4.2.1 Materials and reagents

4.2.2 Analytical instruments and measurements

4.2.3 Fabrication of dye-sensitized solar cells and measurements



4.3.1. Synthesis

4.3.2 Photophysical properties

4.3.3 Electrochemical properties

4.3.4 Photovoltaic properties

4.4 Conclusion

4.5 Reference

Chapter 5. The effects of the number of anchoring groups and

N-substitution on the performance of phenoxazine dyes in dye-

sensitized solar cells

xiii

5.1 Introduction

5.2 Experimental







5.3.2 Density functional theory (DFT) calculations

5.3.3 Photophysical properties of the dyes in solution and on TiO2 film

5.3.4 Electrochemical properties

5.3.5 Photovoltaic properties

5.3.6 Electrochemical impedance spectroscopy

5.4 Conclusion

5.5 References

xiv

Summary

Korean Abstract

List of Publications

List of Presentations

xv

List of Tables

Table 2.1 Solubility of the dyes at 20℃.

Table 2.2 Absorption maxima and extiction coefficients of the prepared dyes in

CH2Cl2 and PB 16 in sulfuric acid.

Table 2.3 Electronic energies of the prepared dyes.

Table 2.4 The coordinate values corresponding to the CIE 1931 chromaticity

diagram of the dye-based black matrix.

Table 2.5 Dielectric constants and constitutions of the dye-based black matrix.

Table 3.1 Absorption maxima and molar extinction coefficients of the prepared

dyes in cyclohexanone.

Table 3.2 Solubility of the dyes in cyclohexanone at 20℃.

Table 3.3 Optical density a of the prepared films.

Table 3.4 Retention rates of film without dye and film C.

Table 3.5 Dielectric constants the dye-based black matrix at 10kHz.

xvi

Table 3.6 Rq of the prepared films.

Table 4.1 Photophysical and electrochemical properties of WS1, WS2 and WS3

dyes.

Table 4.2 Optimized structures, dihedral angles and electronic distributions in

HOMO and LUMO levels of the prepared dyes.

Table 4.3 DSSC performance parameters of POX, WS1, WS2, and WS3.

Table 5.1 Optimized structures, dihedral angles and electronic distributions in


Table 5.2 Photophysical and electrochemical properties of POX, WB, WH1 and

WH2.

Table 5.3 DSSC performance parameters of POX, WB, WH1, and WH2.

Table 5.4 Lifetime calculations of POX, WB, WH1 and WH2.

xvii

List of Schemes

Scheme 2.1 Synthesis of the prepared dyes.

Scheme 3.1 Conventional and BOT structures of LCD.

Scheme 4.1 Synthesis of POX, WS1, WS2, and WS3: (a) 1-iodobutane, NaOH,

DMSO (b) NBS, CHCl3 (c) POCl3, DMF, CHCl3 (d) 2M aqueous of K2CO3,

Pd(pph3)4, THF (e) cyanoacetic acid, piperidine, acetonitrile.

Scheme 5.1 Synthesis of POX, WB, WH1 and WH2.

xviii

List of Figures

Figure 1.1 Basic structure of liquid crystal display.

Figure 1.2 Fundamental structure of liquid crystal display color filter..

Figure 1.3 a) Fundamental processes in a dye-sensitized solar cell b) Energy-

level diagram of a DSSC.

Figure 1.4. Fabrication of the dye sensitized solar cells.

Figure 2.1 Structure of Pigment Blue 16.

Figure 2.2 Geometry-optimized structures of the prepared dyes.

Figure 2.3 Absorption spectra of the synthesized dyes in CH2Cl2(10-5mol litre-1)

and PB 16 in sulfuric acid(10-5mol).

Figure 2.4 Transmittance spectra of the spin-coated black matrix with dye 1b.

Figure 2.5 Thermogravimetric analysis (TGA) of the prepared dyes.

Figure 3.1 Structure of Zn-PC and PER.

Figure 3.2 Absorbtion spectra of Zn-PC and PER in cyclohexanone(10-5molL-1).

xix

Figure 3.3 Differential scanning calorimetry(DSC) measurements of the

prepared dyes.

Figure 3.4 Thermogravimetric analysis (TGA) of the prepared dyes.

Figure 3.5 Transmittance spectrum of film A.

Figure 3.6 Transmittance spectrum of film B ( Zn-PC : PER = 2 mol:1 mol) ,

film C ( Zn-PC : PER = 3 mol:1 mol) and solution (Zn-PC : PER = 1 mol:1

mol).

Figure 3.7 SEM images of film B and film C.

Figure 3.8 AFM images of film B and film C.

Figure 4.1 Structure of POX, WS1, WS2, and WS3.

Figure 4.2 Absorbtion spectra of WS1, WS2, and WS3 in (a) EtOH/CH2Cl2 (7 :

2; v/v) (10-5molL-1) and (b) on TiO2.

Figure 4.3 Dyes’ HOMO and LUMO energy levels.

Figure 4.4 CV curves of Fc/Fc+, POX, WS1, WS2, and WS3 in CH2Cl2.

Figure 4.5 (a) IPCE spectra DSSCs based on POX, WS1, WS2, WS3, and

N719 and (b) the DSSCs' J–V curves under AM 1.5G simulated sunlight (100

xx

mWcm-2).

Figure 4.6 DSSCs' J–V curves based on POX, WS1, WS2, and WS3 in the dark.

Figure 4.7 Impedance spectra of DSSCs based on POX, WS1, WS2, and WS3;

Nyquist plots measured at 0.60 V forward bias in the dark.

Figure 4.8 Electron lifetime of N719, POX, WS1, WS2, and WS3 as a function

of bias voltage.

Figure 5.1 Structure of POX, WB, WH1 and WH2.

Figure 5.2 Absorbtion spectra of POX, WB, WH1 and WH2 in (a) THF (10-

5molL-1) and (b) on TiO2.

Figure 5.3 Dyes’ HOMO and LUMO energy levels.

Figure 5.4 CV curves of Fc/Fc+, POX, WB, WH1 and WH2 in DMF.

Figure 5.5 Emission spectra of POX, WB, WH1 and WH2 in THF.

Figure 5.6 Normalized absorption and emission spectra of POX, WB, WH1 and

WH2 in THF.

Figure 5.7 (a) IPCE spectra DSSCs based on POX, WB, WH1 and WH2 and (b)

the DSSCs' J–V curves under AM 1.5G simulated sunlight (100 mWcm-2).

xxi

Figure 5.8 FT-IR spectra of (a) WB absorbed on TiO2 and (b) WH2 absorbed

on TiO2.

Figure 5.9. DSSCs' J–V curves based on POX, WB, WH1 and WH2 in the dark.

Figure 5.10 Impedance spectra of DSSCs based on POX, WB, WH1, and WH2.

(a) Nyquist plots measured at 0.55 V forward bias in the dark, (b) Bode phase

plots measured under illuminations (AM 1.5G).

Figure 5.11 Electron lifetime of POX, WB, WH1, and WH2 as a function of

bias voltage.

Figure 5.12 (a) Resistance and (b) capacitance of POX, WB, WH1 and WH2 as

a function of bias voltage.

1

Chapter 1

Introduction

1.1 An overview of LCDs (Liquid Crystal Displays)

There exist several types of display system to visualize information such as

cathode ray tubes (CRTs), electroluminescence (EL) devices, field emission

devices (FEDs), plasma display panels (PDPs) and liquid crystal displays

(LCDs). Each of the displays has its own special characteristics and the proper

choice of display for a particular use depends on many factors such as cost, size,

brightness, definition, life, power consumption, temperature range, operating

voltage and device circuit, etc. [1, 2].

Among these display systems, the LCDs have emerged as the most promising

displays during last decade. The extremely low power consumption, low

voltage operation, high definition, compactness and flexibility of size are the

distinctive features which make LCDs preferable over other types of displays

[3,4]. Figure 1.1 shows a basic structure of LCD panel. The LCD is basically

consisted of a thin layer of liquid crystal sandwiched between a pair of

polarizers. To control the optical transmission of the display element

2

electronically, the liquid crystal layer is placed between transparent electrodes

[e.g., indium tin oxide (ITO)]. The thickness of the liquid crystal layer is kept

uniform by using spacers that are made of photosensitive polymers. The

polarizer and the electrodes are cemented on the surfaces of the glass plates. By

applying a voltage across the electrodes, an electric field inside the liquid

crystal can be obtained to control the light transmission through the liquid

crystal cell.

Figure 1.1 Basic structure of liquid crystal display

3

1.2 Structures and requirements of LCD black matrix

Generally, a color filter consists of clear substrate, black matrix (BM), RGB

color layers, overcoat layer, and column spacer. The BM material is coated on

clear substrate in the optically inactive areas to prevent light leakage and

provide a light shield for the amorphous silicon transistors. BM material can be

organic or inorganic and carbon black is the most popular organic choice. The

RGB color layers are fabricated with either dyes or pigments and protection

overcoat layer is deposited over the color layers. A column spacer is a material

used to maintain a uniform cell gap between the TFT and the color filter glass.

The structural configuration of BM films on color filters is shown in Figure 1.2

[3,4].

The BM film plays vital role in blocking the light to TFT and defending the

contrast ratio reduction by photo leak from a non-display area. Therefore, for a

practical use, the BM with a high opacity property or optical density (OD) over

4/μm and a good resolution of 10-30μm is desired. In addition, low reflectance

is also preferred property of BM for LCD applications.

It is also important to increase the aperture ratio of BM for high transmittance.

Fabrication of BM with high aperture ratio can reduce the power consumption

4

of TFT-LCDs by lowering backlight power. Besides, in case of LCDs with a

black matrix-on-thin film transistor (BOT) structure, the dielectric constant of

BMs needs to be <7 for satisfactory industrial application. The high dielectric

constant of BM can cause malfunctions of LCD TFTs due to interference of the

TFT electric signal with BM materials.

In addition, BM must exhibit high heat resistance without thermal flow and

small chromatic changes during the alignment layer formation step. The

chromatic changes (ΔEab) should be less than 3 after heating at 250oC for 1 h

and the retention rate of BM (the ratio between the thickness of the film after

prebaking and post-baking) needs to be >80% for satisfactory thermal stability

[4,5]. The light stability of the pixels is important because BMs are exposed

to a lamp with ultraviolet (UV) filter for more than two million lux. The

chemical stability is also important since BMs are exposed to solvents, acids

and bases during the LCD fabrication process. In detail, the cured film must be

stable when exposed to solvents such as NMP and γ-butyrolactone, and to acids

used in etching process or bases used in the development system [4,5].

5

Figure 1.2 Fundamental structure of Liquid crystal display color filter

6

1.3 Fabrication of black matrix pattern using black matrix photo-

resist

BM pattern is fabricated using a BM photo-resist. The BM photo-resist is a

light-sensitive material used in several industrial processes, such as photo-

lithography and photo-engraving to form a patterned coating on a surface.

Photo-resists are classified into two groups: positive and negative-tone resists.

> A positive-tone photo-resist is a type of photo-resist in which the portion of

the photo-resist that is exposed to light becomes soluble to the photo-resist

developer. The portion of the photo-resist that is unexposed remains insoluble

to the photo-resist developer.

> A negative-tone photo-resist is a type of photo-resist in which the portion of

the photo-resist that is exposed to light becomes insoluble to the photo-resist

developer. The unexposed portion of the photo-resist is dissolved by the

photo-resist developer.

The BM photo-resist is negative-tone photo-resist which is made up of BM

material, dispersant, photo-sensitive binder, multi-functional monomer, photo-

7

initiator and additives such as leveling agent and coupling agent. Conventional

fabrication process of BM is following complicated photolithographic method

using BM photo-resist [4,5] as shown below.

1. Spin coating process;

BM photo-resist is dropped onto a glass substrate to prepare a black film by a

spin coating process.

2. Prebaking process;

The black film is prebaked on a hot plate.

3. Exposure;

To make the pattern insoluble, it is UV cured by exposure through a photo-

mask.

4. Development & postbaking;

After the removal of unnecessary portions of the color resist by the developing

solution, the pattern is cured through postbaking.

8

1.4 An overview of DSSCs (Dye-sensitized Solar Cells)

The generation speed of fossil fuels is much slower than the depletion rate,

leading to human society getting close to an energy and environmental crisis.

Solar energy is regarded as one of the perfect energy resources owing to its

huge reserves, inexhaustibility and pollution-free character. Direct convertion of

solar light to electric energy based on photovoltaics is the optimal way for

electrified modern society.

Different technologies in photovoltaics, such as crystalline Si,

semiconductor(e.g., GaAs)-based cells, thin-film (e.g., CdTe) solar cells,

organic bulk heterojunction (BHJ) solar cells and dye-sensitized solar cells

(DSSCs), co-exist to compete in the future market. Among them, the DSSCs

have been considered as a highly promising and cost-effective alternative for

the photovoltaic energy sector and attracted considerable attention in recent

years.[6]

Dye-sensitized solar cells generate electricity by converting energy from light

absorbed by the dye. Since these solar cells can be produced from low-cost

materials using simple manufacturing processes, overall manufacturing

expenditures are expected to be comparatively low. Other advantages over

9

silicon-based solar cells include the ability to use a variety of designs and colors

and, achieve high performance under indoor and low light settings. In addition,

changes in the angle at which light hits the surface of the cells have minimal

effect on the performance. Such advantages are expected to expand the range of

use for solar cells, which are ideal for a variety of consumer-related applications

in which conventional solar cells are unsuitable.

Presently, organic sensitizers for DSSCs fall into two broad categories:

metal-polypyridyl complexes and pure metal-free organic dyes. DSSCs based

on ruthenium (Ru)-polypyridyl dyes usually show high efficiency (10–11%)

due to their wide absorption range from the visible to the near-infrared (NIR)

regime. However, high production cost and difficulties in purification have

limited their development for large-scale applications. Recently, with focus on

the cost and limited Ru resource, a metal-free organic sensitizer could become

very promising once the efficiency and stability were improved to a viable level.

Compared to Ru dyes, metal-free sensitizers have advantages of low cost and

structural design flexibility. Moreover, the molar extinction coefficients of

metal-free dyes are normally much higher than that of Ru dyes. Undoubtedly,

molecular engineering is the most effective method to improve the performance

from the viewpoint of sensitizers[7]. For these reasons, sensitizers without Ru

10

such as triphenylamine, indoline, cyanine, coumarin, perylene, porphyrin,

phthalocyanine, and phenothiazine have been extensively studied. Among these,

porphyrin derivatives have shown high electronic conversion efficiency

(12.3%).

11

1.5 Operating principle and fabrication of DSSCs

Conventional DSSCs typically contain five components: 1) a photoanode, 2) a

mesoporous semiconductor metal oxidefilm, 3) a sensitizer (dye), 4) an

electrolyte/hole transporter, and 5) a counterelectrode. In DSSCs, the

photosensitizer dye can absorb sunlight and create a high energy state, from

which a photo-excited electron is injected into the conduction band of the TiO2

semiconductor. After percolation through the thin mesoscopic semiconductor

film, the injected electrons are collected by the conducting substrate and flow

into the external circuit. Simultaneously, the oxidized sensitizers generated after

electron injection are reduced to their neutral state by the reducing species

(generally iodide ions) in the electrolyte solution. The oxidized species in the

electrolyte diffuse into the counter electrode and receive the external circuit

electrons to complete the whole circuit. Such a cycle is kept repeating without

any material being consumed but energy being transformed from light to

electricity. The general operating principle of a dye-sensitized solar cell is

depicted in Figure 1.3. [8]

12

Figure 1.3 a) Fundamental processes in a dye-sensitized solar cell b) Energy-

level diagram of a DSSC.

Figure 1.4 depicts fabrication of the dye sensitized solar cells. First, one

prepares glass substrate with a transparent conducting layer. Then, the DSSC

13

working electrodes are prepared. For the paste used in the light-scattering layers,

10nm TiO2 particles which were obtained after the peptization step were mixed

with 400 nm TiO2 colloidal solution. Pastes are coated on FTO, forming the

TiO2 film. It is treated with TiCl4 before sintering. After cooling, the electrode

is immersed in dye solutions. Finishing fabrication of all parts, the dye-covered

TiO2 electrode and Pt-counter electrode are assembled into a sandwich type cell

and sealed with a hot-melt gasket. For the counter electrode, a hole (1-mm

diameter) is drilled in the FTO glass. The hole is made to let the electrolyte in

via vacuum backfilling. After the injection of electrolyte, the hole is sealed

using a hot-melt film and a cover glass.

14

Figure 1.4 Fabrication of the dye sensitized solar cells.

15

1.6 Requirements of organic sensitizers

Dyes employed in highly efficient DSSC have to meet several requirements;

the absorption spectrum on the nanoporous TiO2 layer, the energy level of the

ground/exited state, the charge injection/recombination and the stability. [9]

1) Absorption spectrum

The dye concentration within the nanoporous TiO2 electrode and the

absorption coefficient determine the amount of light that is absorbed. Therefore,

the dye should have a high absorption coefficient in the visible region and a

high affinity to the TiO2 to ensure a high efficiency. With increasing absorbance

of the TiO2-electrode, the layer thickness can be decreased, which is

advantageous for two reasons:

– The recombination probability decreases with decreasing electrode thickness.

– More viscous electrolytes with low vapor pressure can be applied.

In addition, the dye also should have a broad absorption range. If it is possible

to synthesize novel dyes with an absorption that extends into the near infrared

(NIR), the short circuit current can be significantly improved.

16

2) Energy level

The energy level of the exited dye molecule should be about 0.2 - 0.3 eV

above the conduction band of the TiO2 to ensure efficient charge injection. In

this case, the activation energy for the back reaction is also high and the

corresponding rate is too slow to compete with the dye regeneration by the

electrolyte. In addition, the highest occupied molecular orbital of the dye should

lie below the energy level of the hole transporter, so that the oxidized dyes

formed after electron injection into the conduction band of TiO2 can be

effectively regenerated by accepting electrons from the hole transport material.

3) Charge injection and charge recombination

Charge injection occurs from the π*-orbitals of the anchoring group

(carboxylic or phosphonic acid) to the titanium 3d-orbitals. Thus, a good

overlap of these orbitals is mandatory for efficient charge injection. Injection of

electrons from the dye into the TiO2 typically happens on a femto- to

picosecond time scale whereas charge recombination in the micro to

millisecond time scale.

For an efficient charge injection, the regeneration of the sensitizer by a hole

17

transporter should be much faster than the recombination of the conduction

band electrons with the oxidized sensitizer. For example, one finds a

recombination rate of krec = 1.4x103 s-1 and a regeneration rate of kreg =

1.1x105s-1 for the prepared dye. Thus the regeneration is a hundred times faster,

which ensures an injection yield of 99 %.

4) Stability

Any sensitizer in DSSC has to sustain at least twenty years of operation

without significant degradation. Ideally, the electron injection and regeneration

is completely reversibly. However, in any real device, some degradation of the

dye occurs. The standard accelerated aging test lasts typically for 1000 hours,

which corresponds to one year of outdoor application.

18

1.7 References

[1] Yeh P, Gu C. Optics of liquid crystal displays. John Wiley & Sons, Inc; 1999.

P. 1-5.

[2] Yang DK, Wu ST. Fundamentals of liquid crystal devices. New Jersey:

Wiley-SID; 2006: 278–281.

[3] Choi J, Kim SH, Lee WS, Yoon C, Kim JP. Synthesis and characterization

of thermally stable dyes with improved optical properties for dye-based LCD

color filters. New J. Chem. 2012; 36:812–818.

[4] Sabnis RW. Color filter technology for liquid crystal displays. Displays

1999; 20:119–29.

[5] Tsuda K. Color filters for LCDs. Displays 1993;14:115–124.

[6] (a) Hagberg DP, Yum J-H, Lee H, De Angelis F, Marinado T, Karlsson KM.

Molecular engineering of organic sensitizers for dye-sensitized solar cell

applications. Journal of the American Chemical Society 2008; 130: 6259-66

(b) Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and

stable dye-sensitized solar cells with an organic chromophore featuring a binary

π-conjugated spacer. Chemical Communications 2009; 16: 2198-200

[7] (a) O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on

19

dye-sensitized colloidal TiO2 films. Nature 1991; 353: 737-40

(b) Grätzel M. Dye-sensitized solar cells. Journal of Photochemistry and

Photobiology C 2003; 4: 145-53

(c) Grätzel M. Conversion of sunlight to electric power by nanocrystalline dye-

sensitized solar cells. Journal of Photochemistry and Photobiology A 2004;

164(3): 3-14

(d) Park N-G, Kim K. Transparent solar cells based on dye-sensitized

nanocrystalline semiconductors. Physica Status Solidi (a) 2008; 205(8): 1895-

904

[8] (a) Yongzhen W and Weihong Zhu, Organic sensitizers from D–p–A to D–

A–p–A: effect of the internal electron-withdrawing units on molecular

absorption, energy levels and photovoltaic performances. Chem Soc Rev 2013;

42: 2039-58

(b) Mishra A, Fischer MKR, and Bäuerle P, Metal-Free Organic Dyes for Dye-

Sensitized Solar Cells: From Structure: Property Relationships to Design Rules.

2009: 48: 2474-99

[9] (a) Kim S, Lee JK, Kang SO, Ko J, Yum J-H, Fantacci S, et al. Molecular

engineering of organic sensitizers for solar cell applications. Journal of the

American Chemical Society 2006; 128(51): 16701-7

20

(b) Koumura N, Wang Z-S, Miyashita M, Uemura Y, Sekiguchi H, Cui Y, et al.

Substituted carbazole dyes for efficient molecular photovoltaics: long electron

lifetime and high open circuit voltage performance. Journal of Materials

Chemistry 2009; 19: 4829-36

(c) Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK, et

al. Porphyrin-sensitized solar cells with cobalt (II/III)-Based redox electrolyte

exceed 12 percent efficiency. Science 2011; 334: 629-34

21

Chapter 2

Synthesis and characterization of solubility enhanced metal-free

phthalocyanines for liquid crystal display black matrix of low

dielectric constant

2.1 Introduction

The black matrix (BM), a component of LCD color filters, divides the red,

green and blue (RGB) pixels of the color filter and blocks light leaking from the

areas between the RGB color patterns enhancing contrast ratio of color filters.[1]

According to the component materials, BM can be divided into metal oxide,

carbon black, titanium black and organic pigment types.[2] Among the various

types of BMs, a carbon black BM manufactured by spin-coating is generally

employed in industry. The advantage of using carbon black as a BM component

arises from its high light absorption property, high thermal stability and low

cost.[2, 3] On the other hand, malfunctions of the thin film transistor (TFT) of a

LCD can occur due to the high dielectric constant of carbon black. To avoid

this problem, an organic pigment BM with a low dielectric constant can be used

despite its low spectral property due to the lower molar extinction coefficient of

22

organic pigments.[4]

If dyes are used for the manufacture of the BM, the high dielectric constant

and low light absorption of conventional BMs can be overcome. On the other

hand, dyes generally have low thermal stability compared to carbon black and

organic pigments. Therefore, the dyes for BM need to be structurally stable.[5,

6] They also should have good solubility in industrial solvents, such as

propylene glycol methyl ether acetate (PGMEA) and cyclohexanone.[6]

In this study, green phthalocyanine (PC) dyes with high thermal stability and

high solubility were designed. Three metal-free PC dyes were synthesized by

introducing substituents including alkyl or alkoxy groups to the peripheral

position of the PC rings, and their spectral properties, solubilities and thermal

stabilities were measured. Dye-based BMs were fabricated and their optical and

dielectric properties were examined.

23

2.2 Experimental

2.2.1 General

2,5-bis-(1,1-dimethylbutyl)-methoxyphenol, 4-hydroxy-3-tert-butylanisole, 2,4-

bis(1,1-dimethylpropyl)phenol were purchased from TCI and, dimethyl

sulfoxide (DMSO), potassium carbonate anhydrous, dichloromethane, m-

xylene anhydrous, ethanol anhydrous and lithium granule purchased from

Sigma Aldrich were used as received. All reagents and solvents were of

reagent-grade quality and obtained from commercial suppliers. Transparent

glass substrates were provided by Paul Marienfeld GmbH & Co. KG and

acrylic binder LC20160 were supplied by SAMSUNG Cheil industries Inc.

1H NMR spectra were recorded on a Bruker Avance 500 spectrometer at

500MHz using chloroform-d and tetramethylsilane (TMS), as the solvent and

internal standard, respectively. Matrix Assisted Laser Desorption/Ionization

Time Of Flight (MALDI-TOF) mass spectra were collected on a Voyager-DE

STR Biospectrometry Workstation with α -cyano-4-hydroxy-cynamic acid

(CHCA) as the matrix. Fourier transform infrared (FT-IR) spectra were

recorded in the form of solid on a Thermo Scientific Nicolet 6700 FT-IR

24

spectrometer. Elemetal analysis was done on CE Instrument EA1112.

Absorption spectra were measured using a HP 8452A spectrophotometer.

Thermogravimetric analysis (TGA) was conducted under nitrogen at a heating

rate of 10℃min-1 using a TA Instruments Thermogravimetric Analyzer 2050.

Dielectric constants were measured using Edward E306 thermal evaporator and

HP 4294A precision impedance analyzer.

2.2.2 Synthesis

Preparation of 4-(2,5-Bis(1,1-dimethylbutyl)-4-methoxyphenoxy) phthalo

nitrile (1a) 4-nitrophthalonitrile(1g, 5.77mmol) and 2,5-bis-(1,1-dimethyl-

butyl)-methoxyphenol(1.68g, 5.77mmol) were dissolved in dry DMSO(30ml)

and anhydrous K2CO3(1.06g, 7.66mmol) was added in portions during 4h. The

mixture was stirred at 40℃ for 24h under nitrogen atmosphere. After filtering

the reaction mixture, the residue was extracted with CH2Cl2 and dried by rotary

evaporation. Pure product in 91%(2.19g, 5.23mmol) yield was collected by

column chromatography on silica gel using EA/hexane (10:1)mixture as an

eluent. 1H NMR ( 500MHz; CDCl3; Me4Si): δH, ppm 7.69 (1H, d), 7.22 (1H,

s), 7.14 (1H, d), 6.82 (1H, s), 6.63(1H, s), 3.08 (3H, s, -O-CH3), 1.71 (2H, m),

25

1.56 (2H, m), 1.26(12H, d), 1.04 (2H, m), 0.96 (2H, m), 0.81 (3H, t), 0.71 (3H,

t).

Preparation of 4-(3-tert-butyl-4-methoxyphenoxy)phthalonitrile (2a) 2a

(yield 85%) was synthesized following the same procedure for 1a using 2 (1g,

5.54mmol), 4-nitrophthalonitrile(0.96g, 5.54mmol), DMSO(30ml), and

anhydrous K2CO3(1.06g, 7.66mmol). 1H NMR ( 500MHz; CDCl3; Me4Si): δH,

ppm 7.71 (1H, d), 7.26 (1H, s), 7.22 (1H, d), 7.01 (1H, s), 6.77(2H, m), 3.83

(3H, s, -O-CH3), 1.30 (9H, m).

Preparation of 4-(2,4-Bis(1,1-dimethyl-propyl)phenoxy)phthalonitrile (3a) 3a

(yield 82%) was synthesized following the same procedure for 1a using 3 (1.2g,

5.12mmol), 4-nitrophthalonitrile(0.89g, 5.12mmol), DMSO(30ml), and

anhydrous K2CO3(1.06g, 7.66mmol). ). 1H NMR ( 500MHz; CDCl3; Me4Si):

δH, ppm 7.71 (1H, d), 7.35 (1H, s), 7.27 (1H, s), 7.22 (1H, d), 7.18 (1H, d),

6.75 (1H, d), 1.66 (4H, m), 1.30 (12H, d), 0.70 (3H, t), 0.63 (3H, t).

Preparation of 1(4)-Tetrakis(2,5-Bis(1,1-dimethylbutyl)-4-methoxyphenoxy)-

phthalocyaninatozinc(II) (1a).

Preparation of tetrakis(2,5-Bis(1,1-dimethylbutyl)-4-methoxyphenoxy)-

phthalocyanine (1b) 1a(0.9g, 2.38mmol) was dissolved in anhydrous

xylene(50ml) and ethanol(10ml) under nitrogen atmosphere and Li(0.13g,

26

18.7mmol) was added in the solution. The reaction mixture was stirred at 150℃

for 5h. After cooling the solution, the resulting slurry was extracted with

CH2Cl2(100ml)and washed with saturated NaCl solution. The pure product in

54%(0.53g, 0.31mmol) yield was obtained by column chromatography on silica

gel using CH2Cl2as an eluent. Anal. calcd.(%) for C108H138N8O8: C, 77.38;

H, 8.30; N, 6.68; O, 7.64. Found(%): C, 77.36; H, 8.27; N, 6.66; O, 7.69.

MALDI-TOF MS: m/z 1675.9 (100%, [M+2K]+). IR: ν, cm-1 3289 (NH).

Preparation of tetrakis(3-tert-butyl-4-methoxyphenoxy)-phthalocyanine (2b)

2b (yield 41%) was synthesized following the same procedure for 1b using 2a

(1g, 3.26mmol), Li, ethanol(10ml), and anhydrous xylene(50ml). Anal.

calcd.(%) for C76H74N8O8: C, 74.37; H, 6.08; N, 9.13; O, 10.43. Found(%): C,

74.37; H, 6.08; N, 9.12; O, 10.45. MALDI-TOF MS: m/z 1227.1 (100%,

[M+2K]+). IR: ν, cm-1 3289 (NH).

Preparation of tetrakis(2,4-Bis(1,1-dimethyl-propyl)phenoxy)-phthalocya

nine(3b) 3b(yield 49%)was synthesized following the same procedure for 1b

using 3a (1.1g, 3.05mmol), Li, ethanol(10ml), and anhydrous xylene(50ml).

Anal. calcd.(%) for C96H114N8O4: C, 79.85; H, 7.96; N, 7.76; O, 4.43.

Found(%): C, 79.88; H, 8.75; N, 6.33; O, 4.25. MALDI-TOF MS: m/z 1443.7

(100%, [M+2K]+). IR: ν, cm-1 3293 (NH).

27


The ink for a black matrix was composed of the dye (0.01g), cyclohexanone

(4.0g), and LC20160 (14g) as a binder based on acrylate. The prepared dye-

based inks were coated on a transparent glass substrate using a MIDAS System

SPIN-1200D spin coater. The coating speed was initially 100 rpm for 5s, which

was then increased to 200 rpm and kept constant for 20s. The wet dye-coated

black matrix was then dried at 80℃ for 20 min, prebaked at 150℃ for 10 min,

and postbaked at 230℃ for 1h. After each step, the coordinate values of the

black matrix were measured.

2.2.4 Measurement of spectral and chromatic properties

The absorption spectra of the synthesized dyes and the transmittance spectra

of the dye-based BM were measured using a UV-vis spectrophotometer. The

chromatic values were recorded on a color spectrophotometer (Scinco

colormate).

28

2.2.5 Measurement of solubility

Small amounts of the dyes (0.05g) were added to the solvents (0.5g). The

solutions were stirred for 20 min and left to stand for 24 h at room temperature.

Precipitations were visually checked and additional solvents (0.25g) were added

into the solutions to until it made clear solutions. The solubilty of the dye was

recorded as weight percentage of dyes in the clear solutions.


The thermal stability of the synthesized dyes was evaluated by

thermogravimetry (TGA). The prepared dyes were heated to 110℃ and held at

that temperature for 10 min to remove the residual water and solvents. The dye

was then heated to 230℃ and held at that temperature for 60 min to simulate

the processing thermal conditions of color filter manufacturing. The dyes were

finally heated to 500℃ to determine their degradation temperature. The heating

was carried out at the rate of 10℃min-1 under nitrogen atmosphere. To check

the thermal stability of the dyes in black matrix, the fabricated black matrix

were heated to 230℃ for 1 h in a forced convection oven (OF-02GW Jeiotech

29

Co., Ltd.). The color difference values (ΔEab) before and after heating were

measured on a color spectrophotometer (Scinco colormate) in CIE L’a’b’ mode.

2.2.7 Geometry optimization of the synthesized dyes

The geometry and electric structure of the studied dyes are optimized by the

hybrid density functional theory (DFT) method at the PBE/DNP theory level

performed on Materials Studio 5.0 DMol3 program package.


2.3.1. Synthesis of dyes

Three metal-free PCs with enhanced solubility were designed and

synthesized, as shown in Scheme 2.1. Precursors (1a,2a,3a) were synthesized

through a nucleophilic aromatic substitution reaction between nitro

phthalonitriles and phenols including functional groups (1,2,3).[7] Each

reaction was conducted under similar conditions and all products were obtained

30

in high yield over 80%. The structures of the synthesized precursors were

confirmed by 1H NMR.

The metal-free PCs (1b,2b,3b) were synthesized by a cyclotetramerization

reaction of the precursors and purified by column chromatography.[8]

Theoretically, metal-free PCs synthesized by monosubstituted phthalonitriles

can have 4 constitutional isomers: C4h,C2v,Cs,D2h. Assuming a statistical

distribution, The 4 isomers are expected to be mixed in the ratio of 1:2:4:1.[9]

No attempts to separate these isomers were made and all metal-free PCs were

obtained in relatively high yield. The structures of synthesized PCs were

confirmed by MALDI-TOF spectroscopy, FT-IR spectroscopy, and elemental

analysis.

31

Scheme 2.1. Synthesis of the prepared dyes.

32

2.3.2 Solubility

The dyes need to be dissolved in industrial solvents, such as PGMEA and

cyclohexanone, to a concentration of at least 4~5 wt% to be applied for BM.[6]

Generally, non-substituted PCs tend to form various crystal structures due to

molecular interactions caused by their planar structures. Therefore, non-

substituted PCs have low solubility in most organic solvents, which has limited

their applications.[9]

Substituents including bulky alkyl groups or alkoxy groups were introduced

at their peripheral positions to enhance the solubility of PCs. As a result, the

solubility of metal-free PCs increased significantly compared to that of non-

substituted metal-free PC (PB 16). This is due to the steric hindrance between

the planes of the PC molecules caused by the bulky aromatic substituents

rotated out of the plane of the molecule. In addition, as previously mentioned,

the synthesized dyes would exist in a mixture of 4 isomers, which would

increase their solubility due to the unsymmetrical isomers Cs and C2v among

them.[10] Table 2.1 lists the solubility of the PC dyes and PB 16, and Figure

2.1 shows the structure of PB 16.

33

Dyes 1b and 2b, which included alkyl and alkoxy groups at their terminal

positions, showed higher solubility than dye 3b, which contained only alkyl

groups as substituents. In particular, the solubility of dye 1b in cyclohexanone

reached 10wt% due to the anti-aggregation effects between the PC molecules as

well as the affinity between the ether linkages of the dyes and solvent

molecules.[6] The solubility of dye 2b was relatively lower than that of dye 1b

because of its less bulky alkyl substituents. Dye 3b was barely soluble in

industrial solvents due to little affinity between the dyes and solvent molecules.

The solubility of the dyes corresponded to the simulation data optimized by

the Materials Studio 5.0 DMol3 program. Figure 2.2 shows the optimized

structures of the dyes with bulky substituents out of the planar core. The bulky

substituents of dyes 1b and 2b were twisted perpendicular to upward and

downward directions of the molecular plane, whereas those of dye 3b were

twisted toward one side of the molecular plane. Therefore, dyes 1b and 2b were

more soluble in industrial solvents than dye 3b.

34

Table 2.1. Solubility of the dyes at 20℃.

Dye PGMEA Cyclohexanone

1b 4wt% 10wt%

2b 2.2wt% 2.8wt%

3b less than 1.0% less than 1.0%

PB 16 insoluble insoluble

Fig. 2.1. Structure of Pigment Blue 16.

35

Fig. 2.2. Geometry-optimized structures of the prepared dyes.

36

2.3.3 UV-vis absorption spectra

Figure 2.3 and Table 2.2 show the absorption spectra of dye 1b~3b in Ch2Cl2

and PB 16 in sulfuric acid, respectively. To apply a dye to a BM, it should have

strong and broad absorptions in the visible region. The greenish dyes exhibited

absorption maxima in the 706~708nm range, displaying typical Q-band

absorptions in the 600~750nm range as well as B-band absorptions in the

300~450nm range.[11] The characteristic Q and B bands were due to π- π*

transitions in the heteroaromatic 18- π electron system.[12] The dyes showed

similar spectral properties, indicating that the conjugations of the dyes were not

much affected by the change in introduced substituents.[11] The molar

extinction coefficients of the dyes were approximately 140000~150000, which

significantly exceeded those of the pigments. Therefore, the amounts of the

dyes needed for BM can be reduced.

These results corresponded to the simulation data optimized by the Materials

Studio 5.0 DMol3 program. As shown in Table 2.3, the orbital lobes and

HOMO-LUMO energy gaps of the dyes were almost identical. Accordingly, the

changes in substituents did not affect the absorption maxima of the dyes. Figure

2.4 shows the transmittance spectra of the dye-based BM fabricated with the

37

dye 1b. The fabricated BM absorbed light broadly in the 400~450nm and

600~700nm ranges. Therefore, compensatory dyes absorbing light in the

450~600nm range will be needed for complete absorption of light in the visible

range.

400 500 600 700 8000.0

0.5

1.0

1.5

Abs

orb

ance

Wavelength(nm)

1b 2b 3b PB 16

Fig. 2.3. Absorption spectra of the synthesized dyes in CH2Cl2(10-5mol litre-1) and PB 16 in sulfuric acid(10-5mol).

38

Table 2.2. Absorption maxima and extiction coefficients of the prepared dyes in

CH2Cl2 and PB 16 in sulfuric acid.

Dye λmax (nm) εmax (L mol-1 cm-1)

1b 708 140875

2b 706 142473

3b 706 152855

PB 16 774 10756

39

Table 2.3. Electronic energies of the prepared dyes.

1b 2b 3b

HOMO+3 -5.061 -5.086 -5.506

HOMO+2 -4.972 -5.002 -5.258

HOMO+1 -4.958 -4.934 -5.233

HOMO -4.432 -4.445 -4.501

∆E 1.316 1.318 1.326

LUMO -3.116 -3.127 -3.175

LUMO-1 -3.076 -3.093 -3.133

LUMO-2 -1.722 -1.733 -1.783

LUMO-3 -1.515 -1.531 -1.573

∆E means the energy gap difference between HOMO and LUMO

40

400 500 600 7000

20

40

60

80

100

Tra

nsm

ittan

ce

Wavelength

Fig. 2.4. Transmittance spectra of the spin-coated black matrix with dye 1b.

41

2.3.4 Thermal properties

Phthalocyanines are highly stable dyes due to the strong π-π stacking

interactions from their planar structures.[13] In addition, their high molecular

weight is favorable for intermolecular interactions, such as Van der waals

forces. On the other hand, phthalocyanines including substituents show reduced

stability due to anti-aggregation effects.

For the current LCD manufacturing process, dye molecules need to be stable

up to 230℃.[6] As shown in Figure 2.5, dyes 1b and 2b showed < 1% weight

loss after 1hour at 230C, whereas dye 3b showed approximately 10% weight

loss and degraded gradually with increasing temperature. This was attributed to

the initial decomposition of phthalocyanines from the terminal groups at

230~250C. As PC dyes containing alkoxy groups as substituents were reported

to be more stable than those containing terminal alkyl groups, dye 1b and 2b

showed higher stability than dye 3b.[14]

To measure the thermal stability of BM film, dye 1b was spin coated on a

glass and the ΔEab value of the film was measured. As shown in Table 2.4, the

ΔEab value of the film was 3.434 after heating for 90 minutes at 230℃. This

suggests that the original color of the BM was well maintained due to little

42

degradation of the dye.[15] The thermal stability of dye-based BMs would be

improved further if a commercial binder, adjusted for the pigments, can be

optimized to the dye.[16]

0 100 200 300 4000

10

20

30

40

50

60

70

80

90

100

Wei

ght(

%)

Temperature

1b 2b 3b

Fig. 2.5. Thermogravimetric analysis (TGA) of the prepared dyes.

43

Table 2.4. The coordinate values corresponding to the CIE 1931 chromaticity

diagram of the dye-based black matrix.

Black

Matrix L a b ΔEab

1b

prebake 74.6417 -65.6547 6.851

3.434

postbake 77.3913 -66.2442 8.824

2.3.5 Dielectric properties

The dielectric constant of BM needs to be < 7 for satisfactory industrial

applications with a current thickness of BM (1-1.5μm). On the other hand,

carbon black BM has a high dielectric constant > 20, which can cause

malfunctions of LCD TFTs due to interference of the TFT electric signal with

carbon black.[4] Therefore, to reduce the dielectric constant of BM, dye-based

BM and carbon black-dye hybrid type BMs were fabricated and their dielectric

properties were tested.

Table 2.5 lists the dielectric constants and constitutions of the prepared BMs.

The 6 hybrid type BMs were fabricated with varied compositions of dye 1b and

44

carbon black. The BM 1b-00 prepared with carbon black only had the highest

dielectric constant. The dielectric constant of BM decreased significantly with

decreasing carbon black and increasing dye concentration due to the low

dielectric character of the dye. In particular, BM 1b-100 prepared with the dye

only had the lowest dielectric constant of 3.99 at a frequency of 1000kHz. In

addition, the dielectric constants of the samples were a function of the applied

electric field frequency. The dielectric constant of BM decreased gradually with

increasing frequency from 0.1kHz to 1000kHz. This was attributed to the lag in

charge transfer inside the dye molecule caused by the rapid change in the

external electronic field.[17] Among the 6 BMs prepared, those containing

more than 30wt% of dye had a dielectric constant < 7 at the working frequency

range (100~300Hz).

45

Table 2.5. Dielectric constants and constitutions of the dye-based black matrix.

Frequency(kHz) Dielectric constant(εr, average)

1b-00 1b-20 1b-40 1b-60 1b-80 1b-100

0.1 26.39 20.07 13.94 6.90 6.92 4.96

1 25.47 19.44 13.56 6.78 6.81 4.84

10 20.65 16.10 11.60 6.16 6.25 4.23

100 18.72 14.78 10.89 5.95 6.04 4.12

200 18.22 14.41 12.61 5.87 5.96 4.08

500 17.57 13.94 10.44 5.78 5.86 4.02

700 17.34 13.77 10.35 5.74 5.83 3.99

1,000 17.11 13.61 10.25 5.70 5.79 3.99

Thickness 1.2013 ㎛

Electrode radius 280, 285, 280 ㎛

46

1b-00 1b-20 1b-40 1b-60 1b-80 1b-100

binder 50 wt% 50 wt% 50 wt% 50 wt% 50 wt% 50 wt%

carbon

black

50 wt% 40 wt% 30 wt% 20 wt% 10 wt%

Dye 1b 10 wt% 20 wt% 30 wt% 40 wt% 50 wt%

2.4 Conclusions

Three solubility enhanced phthalocyanine dyes were synthesized, and the dye-

based BMs were fabricated with the most soluble dye. The increase in solubility

of the prepared dyes was attributed to bulky functional substituents at the

peripheral positions of them. Since all dyes had high molar extinction

coefficients, dye-based BMs absorbed light in the visible region with the small

amounts of the dyes. In addition, the dyes including terminal alkoxy groups

showed suitable thermal stability for commercial use due to terminal alkoxy

groups are stable at postbaking temperature. The dielectric constants of the

47

BMs containing more than 30wt% of dyes were significantly lower than that of

the BM prepared with carbon black only.

2.5 References

[1] Chang SC. Improving pattern precision of chromium based black matrix by

annealing. Appl. Surf. Sci. 2008; 254: 2244-2249

[2] Koo HS, Chen M and Kawai T. Improvements in the optical and

characteristics of black matrix films containing carbon nanotubes on color

filters. Diamond Relat. Mater. 2009; 18: 533-536

[3] Kuo KH, Chiu WY, Hsieh KH and Don TM. Novel UV-curable and alkali-

soluble resins for light-shielding black matrix application. Eur. Polym. J. 2009;

45: 474-484

[4] Jung J, Park Y, Jaung JY and Park J. Synthesis of new single black

pigments based on azo and anthraquinone moieties for LCD black matrix. Mol.

Cryst. Liq. Cryst. 2010; 529: 88-94

[5] Sigiura T. Dyed color filters for liquid-crystal displays. J. Soc Inf. 1993; 1:

177-180

[6] Choi J, Sakong C, Choi JH, Yoon C and Kim JP. Synthesis and

48

characterization of some perylene dyes for dye-based LCD color filters. Dyes

Pigm. 2011; 90: 82-88

[7] Aĝritaş MS. Non-aggregating phthalocyanines with bulky 2,4-di-tert-

butylphenoxy-substituents. Dyes Pigm. 2007; 74: 490-493

[8] Cheng G, Peng X, Hao G, Kennedy OV, Ivanov IN, Knappenberger K, Hill

TJ, Rodgers MAJ and Kennedy ME. Synthesis, photochemistry, and

electrochemistry of a series of phthalocyanines with graded steric hinderance. J.

Phys. Chem. A; 107: 3503-3514

[9] Erdoĝmuş A and Nyokong T. Novel, soluble, fluxoro functional substituted

zinc phthalocyanines; synthesis, characterization and photophysicochemical

properties. Dyes Pigm. 2010; 86: 174-181

[10] Chen Y, Hanak M, Blau WJ, Dini D, Liu Y, Lin Y and Bai J. Soluble

axially substituted phthalocyanines: synthesis and nonlinear optical response. J.

Mater. Sci. 2006; 41: 2169-2185

[11] Brewis M, Clarkson GJ, Humberstone P, Makhseed S and Mckeown NB.

The Synthesis of some phthalocyanines and naphthalocyanines derived from

sterically hindered phenols. Chem. Eur. J. 1998; 4: 1633-1640

[12] Tau P and Nyokong T. Electrochemical characterization of tetra- and octa-

substituted oxo(phthalocyaninato)titanium(IV) complex. Electrochim. Acta.

49

2007; 52: 3641-3650

[13] Kobayashi N. Design, synthesis, structure, and spectroscopic and

electrochemical properties of phthalocyanines. Bull. Chem. Soc. Jpn. 2002; 75:

1-19

[14] Hacıvelioğlu F, Durmuş M, Yeşilot S, Gürek AG, Kılıç A and Ahsen V.

The synthesis, spectroscopic and thermal properties of

phenoxyclotriphosphazenyl-substituted phthalocyanines. Dyes Pigm. 2008; 79:

14-23

[15] Kim YD, Kim JP, Kwon OS and Cho IH. The synthesis and application of

thermally stable dyes for ink-jet printed LCD color filters. Dyes Pigm. 2009; 81:

45-52

[16] Yoon C, Choi JH and Kim JP. Synthesis and examination of polymers to

improve pattern clarity and resistance properties of phthalocyanine color pixels

in liquid crystal display. Bull. Korean. Chem. Soc. 2011; 32: 1-4

[17] Tilley R. Colour and the optical properties of materials (1st edn). John

Wiley: New York, 2000; 30-31

50

Chapter 3

Analysis and characterization of dye-based black matrix film of

low dielectric constant containing phthalocyanine and perylene

dyes

3.1 Introduction

Color filters (CF) are one of the important components in LCD devices, and

can also be applied to light-emitting diode (LED) displays and organic light

emitting diode (OLED) displays.[1, 2] The black matrix is a key component of

LCD color filter, and it divides the red, green and blue pixels of the CF, and

blocks light leaking from the areas between the RGB color patterns, enhancing

contrast ratio.[3] In addition, BM prevents malfunction of thin film transistors

(TFT) by minimizing the external incident light, which induces interference in

the TFT electric signal.[4]

In the conventional structure of an LCD, as shown in Scheme 3.1, low

aperture ratio caused by a wide width of the BM pattern limits the picture

quality of the LCD. Therefore, to enhance picture quality and simplify the LCD

structure, BM-on-TFT (BOT) structure is required, in which the BM is located

51

on the TFT array.[2] In LCDs with BOT structure, higher aperture ratio due to

relatively narrow BM width allows more back light transmission, enhancing

brightness and visibility. Materials of low dielectric constant are required

instead of conventional BM materials such as Cr/CrOx and carbon black, since

the CF is directly layered on the TFT in BOT structure.[4]

The most frequently used material for BMs is carbon black, which has

advantages of high thermal stability and high light absorption. However, BMs

fabricated with carbon black have high dielectric constants due to extended π-

conjugation length in the carbon black molecules, causing electrical signal

transduction errors on TFTs.[4] As shown in previous research, dyes have low

dielectric constants compared with carbon black, and show much higher light

absorption properties than organic pigments.[5] Therefore, dyes with high

solubility in industrial solvents and high thermal stability can be excellent

candidates for BMs of low dielectric constant.

BM can be fabricated with a single black material, or a mixture to make the

black. As dyes generally have sharp absorption ranges, dye-based BMs need to

be fabricated by material mixture methods. Dye-based BMs can be fabricated

by mixing red, green and blue (RGB) dyes, or cyan, magenta and yellow (CMY)

dyes, as well as by mixing dyes and carbon black.

52

In this study, greenish Zn-PC dyes and reddish PER dyes were synthesized

and employed to fabricate BMs with low dielectric constant and light

absorption in the whole visible region. The spectral, and thermal properties and

solubility of the prepared dyes were investigated, and optical, thermal and

dielectric properties of the dye-based BMs were examined. For further

investigation of surface morphology of the BM films, dye-based BMs were

probed by Field FE-SEM and AFM.

Conventional structure of LCD

BOT structure of LCD

Scheme 3.1. Conventional and BOT structures of LCD.

53

3.2 Experimental

3.2.1 General

1,8-Diazabicyclo-7-undecene (DBU), 3(4)-nitrophthalonitrile, 2,5-bis-(1,1-

dimethylbutyl)-methoxyphenol and isoquinoline were purchased from TCI, and

ZnCl2, perylene-3,4,9,10-tetracarboxylic dianhydride, 2,6diisopropylaniline, m-

cresol, iodine, sulfuric acid, bromine, acetic acid, potassium carbonate

anhydrous, phenol and 4-tert-butylphenol were purchased from Sigma-Aldrich

and used as received. All the other reagents and solvents were of reagent-grade

and obtained from commercial suppliers. Transparent glass substrates were

provided by Paul Marienfeld GmbH & Co. KG, and acrylic binder was supplied

by NDM Inc.

1H NMR spectra were recorded on a Bruker Avance 500 spectrometer at

500MHz using chloroform-d and tetramethylsilane (TMS), as the solvent and

internal standard, respectively. Matrix-Assisted Laser Desorption/Ionization

Time Of Flight (MALDI-TOF) mass spectra were collected on a Voyager-DE

STR Biospectrometry Workstation with α-cyano-4-hydroxy-cynamic acid

54

(CHCA) as the matrix. Absorption spectra were measured using an HP 8452A

spectrophotometer. Differential scanning calorimetry (DSC) was conducted in

nitrogen atmosphere at a heating rate of 10K/min using a TA instrument

Differential Scanning Calorimeter Q1000. Thermogravimetric analysis (TGA)

was conducted in nitrogen atmosphere at a heating rate of 10℃min-1 using a

TA Instruments Thermogravimetric Analyzer 2050. Dielectric constants were

measured using an Edward E306 thermal evaporator and an HP 4294A

precision impedance analyzer. The thickness of the dye-based BM was

measured using a KLA-TENCOR Nanospec AFT/200 alpha step.

3.2.2 Synthesis

3.2.2.1 2(3)-Tetrakis(2,5-Bis(1,1-dimethylbutyl)-4-methoxyphenoxy)-

phthalocyaninatozinc(II) (Zn-PC).

MALDI-TOF MS: m/z 1738.44 (100%, [M+2K]+). C106H132N8O8Zn Calcd. C,

74.38; H, 7.77; N, 6.55; O, 7.48% Found. C, 74.61; H, 8.04; N, 6.59; O, 7.35%.

3.2.2.2 N,N’-Bis(2,6-diisopropylphenyl)-1-p-tert-butylphenoxy-perylene-

55

3,4,9,10-tetracarboxydiimide (PER)

1H NMR (500Mhz, CDCl3): δ(ppm) 9.68 (d, 1H), 8.82 (m, 2H), 8.75 (m, 3H),

8.42 (s, 1H), 7.48 (m, 4H), 7.33 (m, 4H), 7.13 (d, 2H), 2.74 (septet, 4H), 1.36 (s,

9H), 1.16 (d, 24H). MALDI-TOF MS: m/z 860.28 (100%, [M+2K]+).


The ink for the black matrix was composed of the dyes (0.014 or 0.033g),

cyclohexanone (0.28 g), and acrylic binder (0.12g). The prepared dye-based

inks were coated onto a transparent glass substrate using a MIDAS System

SPIN-1200D spin coater. The coating speed was initially 100 rpm for 5 s,

which was then increased to 500 rpm and kept constant for 20 s. The wet dye-

coated black matrix was then dried at 80℃ for 20 min, prebaked at 150℃ for

10 min, and post-baked at 230℃ for 1h.

3.2.4. Measurement of spectral and optical properties

56

The absorption spectra of the synthesized dyes and the transmittance spectra

of the dye-based BM were measured using a UV-vis spectrophotometer. The

optical density values were calculated from the transmittance spectra at 550nm

and the thickness of the dye-based BMs.

3.2.5 Investigation of solubility

The prepared dyes were added to cyclohexanone at various concentrations,

and the solutions were sonicated for 5 min using an ultrasonic cleaner

ME6500E. The solutions were left to stand for 48 h at room temperature, and

checked for precipitation to determine the solubility of the dyes.


The thermal stability of the synthesized dyes was evaluated by DSC and

57

TGA. In DSC measurements, the prepared dyes were heated up to 250℃ and

held at this temperature for 3 min before cooling to room temperature. In

TGA measurements, the prepared dyes were heated to 110℃ and held at that

temperature for 10 min to remove residual water and solvents. The dyes were

then, heated to 220℃ and held at that temperature for 30 min to simulate the

thermal processing conditions of color filter manufacturing. The dyes were

finally heated to 400℃ to determine their degradation temperature. The

temperature was raised at a rate of 10℃min-1 in nitrogen atmosphere. To check

the thermal stability of the dyes, the fabricated black matrices were heated to

230℃ for 1 h in a forced convection oven (OF-02GW Jeiotech Co., Ltd.). The

differences in thickness values before and after heating were measured using a

KLA-TENCOR Nanospec AFT/200 alpha step.

3.2.7 Field emission scanning electron microscopy and Atomic force

microscopy

The aggregation morphology of the dyes in black matrix was investigated

58

using a field emission scanning electron microscope (JSM 6700-F). The dye

aggregates were observed at an acceleration voltage of 10kV and working

distance (WD) of 6mm, and FE-SEM images of the dye-based black matrix

were taken at 7000x magnification. The surface morphology of the dye-based

BM (10µm x 10µm size) was observed using atomic force microscopy (Park

science) in non-contact mode.


3.3.1 Properties of dyes

As shown in Figure 3.1, previously reported Zn-PC and PER dyes were

synthesized as BM components by following reference synthetic routes.[6,7]

No attempts to separate the mixture of 4 constitutional isomers of Zn-PC

(C4h,Cs,C2v,C4h=12.5:50:25:12.5) were made.[8] The synthesized dyes were

confirmed by H1 NMR, MALDI-TOF spectroscopy and elemental analysis.

59

Zn-PC

PER

Fig. 3.1. Structure of Zn-PC and PER.

Figure 3.2 and Table 3.1 show the absorption spectra of Zn-PC and PER dyes

in cyclohexanone, the industrial solvent of the LCD color filter. To apply dyes

to the BM, they need to have broad and strong absorptions in the visible region.

60

The greenish Zn-PC dye displayed Q-band absorption in the 550-700 nm range,

as well as B-band absorption in the 400-500 nm range, with an absorption

maximum at 684 nm. The typical Q and B bands were due to π- π* transitions

in the heteroaromatic 18- π electron system, and the spectra of the dye were not

much affected by the peripherally introduced substituents at the β position of

the dye.[9] The reddish PER dye exhibited absorption in the 450-570 nm range

with an absorption maximum at 528nm. The PER dye showed a slight

bathochromic shift due to the electron donating power of the mono-substituted

phenoxy moeity at the bay position of the dye.[10,11] As shown in Figure 3.2,

the mixture of the two dyes in cyclohexanone nearly absorbed light in the

whole visible region by combining the absorption ranges of each. The molar

extinction coefficients of Zn-PC and PER dyes were 193588 and 62991,

respectively, which are much higher than those of organic pigments. Therefore,

the amounts of the dyes needed for BM can be decreased compared with

organic pigments.

61

400 500 600 7000.0

0.5

1.0

1.5

2.0

Ab

sorb

an

ce (

AU

)

Wavelength (nm)

Zn-PC PER Zn-PC + PER

Fig. 3.2. Absorbtion spectra of Zn-PC and PER in cyclohexanone(10-5molL-1).

Table 3.1. Absorption maxima and molar extinction coefficients of the prepared

dyes in cyclohexanone.

Dye λmax (nm)

εmax

(L mol-1 cm-1)

Zn-PC 684 193588

PER 528 61991

62

The thermal properties of the dyes were investigated with TGA and DSC, as

shown in Figure 3.3 and Figure 3.4. For dyes to be used as BM, dye molecules

should be stable up to 230 C for current industrial processes.[12,13] During the

heating and cooling process in DSC measurements, no phase transitions due to

melting or changing crystal structures were observed up to 250C. In TGA

measurements, the dyes showed less than 2 % weight loss after 1h at 230 C.

0 50 100 150 200 250-1.5

-1.0

-0.5

0.0

0.5

1.0

He

at F

low

(W

/g)

Temperature ( oC)

Zn-PC

0 50 100 150 200 250-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

He

at F

low

(W/g

)

Temperature ( oC)

PER

Fig. 3.3. Differential scanning calorimetry(DSC) measurements of the prepared

dyes.

63

0 100 200 300 40080

90

100

Wei

gh

t (%

)

Temperature (oC)

Zn-PC PER

Fig. 3.4. Thermogravimetric analysis (TGA) of the prepared dyes.

These results were attributed to the fact that the dyes have mostly planar

molecular structures including plenty of aromatic rings. Therefore this

contributes to strong intermolecular π-π stacked interactions, resulting in the

high thermal stability of the dyes.[14,15,16] In addition, intermolecular

interactions due to their high molecular weight and polar functional substituents

increase thermal stability of the dyes.[17] From the TGA and DSC

measurements, it is concluded that the dyes can endure LCD manufacturing

process without phase transitions and degradations.

Table 3.2 lists the solubility of the dyes in cyclohexanone. The dyes need to

be dissolved in industrial solvents to a concentration of at least 5wt% to be

64

applied for BM. The solubilities of Zn-PC dye and PER dye in cyclohexanone

were 8wt% and 10wt%, respectively. The high solubility of the dyes was

presumed to be due to the affinity between the ether linkages of the dyes and

cyclohexanone molecules. In addition, bulky and polar substituents reduced the

planarity of the dyes, enhancing their solubility. In particular, the substituents at

the bay position of the PER dye induced twisting of the naphthalene subunits in

the perylene core, reducing intermolecular aggregations and resulting in higher

solubility in cyclohexanone than Zn-PC dye.[18,19]

Table 3.2. Solubility of the dyes in cyclohexanone at 20℃.

Zn-PC PER

Cyclohexanone 8 wt% 10 wt%

65

3.3.2. Spectral and optical properties of dye-based black matrix

The dyes in the film state should have broad and strong absorptions in the

visible region. To investigate the light blocking property of the dye-based BM,

film A (dye content : 7.6wt%, thickness : 12.5µm ) was prepared with the Zn-

PC and PER dyes, in which the dyes were used to the extent of maximum

solubility in cyclohexanone. The transmittance spectra of film A is shown in

Figure 3.5. The prebaked Film A blocked the light in the whole visible region,

except for 3% transmittance around 450nm and 6% transmittance around

580nm. The transmittance spectra of film A after post-baking was similar to that

of film A after prebaking, except for a small increase in the transmittance

around 580 nm. The high transmittance regions are consistent with the regions

where both dyes have weak light absorptions.

To investigate the optical properties of film A, its optical density (OD) was

measured and listed in Table 3. The OD can be expressed as follows.

OD = (log1/T)/d

where T and d are the transmittances at 550nm and the thickness of film,

66

respectively.[20]

400 500 600 7000

10

20

30

40

50

60

70

80

90

100

Tra

nsm

ittan

ce (

%)

Wavelength (nm)

After postbake After prebake

Fig. 3.5. Transmittance spectrum of film A.

Table 3.3. Optical density a of the prepared films.

After prebaking After postbaking

Film A 0.233/ µm 0.226/ µm

Film B 0.085/ µm 0.083/ µm

Film C 0.095/ µm 0.092/ µm

a Optical density was investigated at 500nm

67

According to this, the OD of the film after prebaking and post-baking were

0.233/µm and 0.226/µm, respectively. In general, the OD of the BM film needs

to be >2.5/µm for satisfactory industrial applications.[21] Therefore, for high

OD of the dye-based film, the dye content in the solution has to be increased,

and dispersants or surfactants might have to be used for this purpose. If the dye

content in the film is increased up to the same content of carbon black or

organic pigments in the conventional BM (50wt% of the BM resist), the

calculated OD of dye-based BM would be 1.78/µm. This OD value is higher

than that of organic pigment (~1.2/µm), while it is still lower than that of

carbon black. Insufficient OD of dye-based BM can be enhanced by the

fabrication of a hybrid BM, which includes dye and carbon black together. In a

dye-carbon black hybrid BM, carbon black can also compensate for the narrow

absorption ranges of the dye, preventing light leaking in visible regions.

68

400 500 600 7000

20

40

60

80

100

Tra

nsm

ittan

ce (

%)

Wavelength (nm)

After postbaking After prebaking Solution

Film B

400 500 600 7000

20

40

60

80

100

Tra

nsm

ittan

ce (

%)

Wavelength (nm)

After postbaking After prebaking Solution

Film C

Fig. 3.6. Transmittance spectrum of film B ( Zn-PC : PER = 2 mol:1 mol) , film

C ( Zn-PC : PER = 3 mol:1 mol) and solution (Zn-PC : PER = 1 mol:1 mol).

69

To investigate the spectral and optical properties of the BM film after the

baking process, thin dye-based BMs with low dye content were prepared. By

varying the ratio of Zn-PC to PER dyes, film B ( dye content : 3.5 wt%, Zn-PC :

PER = 2mol : 1mol, thickness : 7.84 µm ) and film C ( dye content : 3.7 wt%,

Zn-PC : PER = 3mol : 1mol, thickness : 8.50 µm ) were fabricated, and their

transmittance spectra were measured. As shown in Figure 3.6, the films after

post-baking exhibited increased transmittance values around 450nm and 580nm

compared with the films after prebaking. In particular, marked increases in

transmittance were shown in the 620-680nm range, where the Q-band of the

Zn-PC dye is located. This is considered to occur due to the aggregations

between Zn-PC dye molecules after post-baking. It is known that aggregates of

Zn-PC dye show additional absorption peaks before the main Q band.[22,23,24]

As shown in Figure 3.6, this phenomenon was confirmed by the low

transmittance around 650nm of the post-baked films in comparison with the

solution state, which meant that additional aggregation absorption peaks were

formed after post-baking. Further proof of the Zn-PC dye aggregation is

discussed in chapter 3.5. In the case of PER dyes, no definite dye aggregations

after post-baking were deduced from the small changes of OD values and

transmittances around 530nm during the baking process. The OD values of film

70

A, film B and film C are listed in Table 3.3.

3.3.3 Thermal properties of dye-based black matrix

For the current LCD manufacturing process, the BM film needs to be stable

up to 230 C.[6] To test the thermal stability of the dye-based BMs, film C and

the BM film without dyes were prepared and their retention rates were

measured.

The retention rate of BM is defined as the ratio between the thickness of the

film after prebaking and post-baking, and it needs to be >80% for satisfactory

thermal stability. A 20% decrease of the film thickness was observed, mainly

due to solvent vaporization during the post-baking process, as the boiling point

of the solvent in this system is lower than the manufacturing temperature

(230C), and binders are known to be stable at this temperature.[20] As shown

in Table 3.4, the retention rates of film C and film without dye were 80% and

83%, respectively. The retention rate of the dye-based film was almost the same

as that of the film without dye, indicating that the dye-based film had sufficient

71

thermal stability. The results were attributed to the high thermal stability of the

dye molecules, which was confirmed by TGA and DSC measurements. This

was consistent with the results that changes of transmittance and OD between

films after prebaking and post-baking were only due to aggregations of the dyes.

Table 3.4. Retention rates of film without dye and film C.

Retention rate (%)

Film without dye 83

Film C 80

3.3.4. Dielectric properties of dye-based black matrix

The dielectric constant of BMs with BOT structure needs to be <7 for

satisfactory industrial application. However, the carbon black BM has a high

dielectric constant of >20, which can cause malfunction of LCD TFTs with

72

BOT structure due to interference of the TFT electric signal with carbon

black.[4] In order to investigate the dielectric property of the dye-based BM,

film B(dye content : 3.5wt%, dielectric constant : 2.0) and film C(dye content :

3.7wt%, dielectric constant : 2.63) were prepared, and their dielectric constants

at a frequency of 10kHz were measured. The dielectric constants of the

prepared films and the previously reported film (dye content : 50wt%, dielectric

constant : 4.23) are listed in table 3.5.[5] The dielectric constants of all films

were <7, though the dye amount of the previously reported film was fifteen

times greater than those of films B and C. Therefore, the dye-based BMs have

sufficiently low dielectric constants regardless of the type and amount of dye. In

addition, at the working frequency range (100-300 kHz), the dielectric constants

of the BM can be further reduced due to lag in the charge transfer in the dye

molecules.[25] This low dielectric character of dye makes it possible to

manufacture hybrid-type BMs that include dye and carbon black. Hybrid-type

BMs containing 20wt% carbon black and 30wt% dye is considered to have a

dielectric constant of <7 with good overall light absorption properties.

73

Table 3.5. Dielectric constants the dye-based black matrix at 10kHz.

Film B film C

Previously

Reported film

Dielectric

constant

(εr, average)

2.00 2.63 4.23

3.3.5. Surface investigation of BM film by FE-SEM and AFM

In order to investigate the surface morphology of the films after prebaking

and post-baking, film B and film C were studied by using FE-SEM and AFM.

The FE-SEM and AFM images are shown in the Figure 3.7 and Figure 3.8,

respectively and the Rq(root mean square average roughness) values of the

films are listed in Table 3.6.

74

Film B after prebaking

Film B after postbaking

75

Film C after prebaking

Film C after postbaking

Fig. 3.7. SEM images of film B and film C.

76

As shown in FE-SEM images, the aggregation of the dyes on both films after

post-baking increased compared to that after prebaking. As discussed earlier,

this aggregation was attributed to the stacking of Zn-PC dyes, and accordingly,

film C, which contains more Zn-PC dyes, showed the higher aggregation. These

results are thought to arise from the differences in the molecular structure and

solubility of Zn-PC and PER dye. The substituted moiety at the bay position of

PER dye twists the core of the dye, breaking its planarity.[19] However, the

planar structure of Zn-PC dye was maintained in spite of substitutions at its

peripheral positions. Therefore the strong π- π interactions between the planes

of Zn-PC dye molecules were still formed, giving more aggregates. In addition,

Zn-PC dyes have a higher tendency to aggregate during the evaporation of

solvents due to the lower solubility in industrial solvents.

The aggregates on the films are shown three-dimensionally in the AFM

images. In films B and C after post-baking, aggregates were grown

perpendicular to the plane of the films. This was due to the disk-like shapes of

Zn-PC dyes, which led to columnar structures.[26] From the Rq values of the

films, surface roughness of the films can be inferred; generally, films with Rq

value between 1.3nm and 25nm are known to have smooth surfaces.[27] The

Rq values of film B after prebaking and post-baking were 1.52nm and 2.30nm,

77

respectively, and the Rq values of film C after prebaking and post-baking were

1.97nm and 2.61nm, respectively. Though the films after post-baking have

increased Rq values, they are considered to have sufficiently smooth faces for

application in for BMs. Also, no definite phase separations were observed in

microscopic images, which meant that dyes were properly blended except for

dye aggregations.

78

Film B after prebaking

Film B after postbaking

79

Film C after prebaking

Film C after postbaking

Fig. 3.8. AFM images of film B and film C.

80

Table 3.6. Rq of the prepared films.

film B Film C

After

prebaking

After

postbaking

After

prebaking

After

postbaking

Rq 1.52 2.30 1.97 2.61

3.4 Conclusion

The dye-based BM films were fabricated with greenish phthalocyanine and

reddish perylene dyes. The high thermal stability of the dye-based BM was

attributed to the rigid molecular structures of the dyes. In addition, due to the

low dielectric characteristics of the dye, the dielectric constants of the dye-

based BMs were significantly lower than that of the BM prepared with carbon

black only. However, the low solubility of the dyes in industrial solvents and

dye aggregations in the baking process limited the input of the dye in the BM

resist, resulting in low light absorption of the dye-based BM. By fabricating

hybrid-type BM that includes dye and carbon black together, the light

81

absorption property of the BM would be improved compared to the dye-based

BM, satisfying the property requirements of BMs.

3.5 References

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[3] Chang SC. Improving pattern precision of chromium based black matrix by

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[4] Jung J, Park Y, Jaung JY and Park J. Synthesis of new single black

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[5] Lee W, Yuk SB, Choi J, Jung DH, Choi S, Park J and Kim JP. Synthesis and

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[9] Brewis M, Clarkson GJ, Humberstone P, Makhseed S and Mckeown NB.

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[10] Langhals H, Blanke P. An approach to novel NIR dyes utilising a-effect

donor groups. Dyes Pigm. 2003; 59: 109−116.

[11] Zhao C, Zhang Y, Li R, Li X, Jiang J. Di(alkoxy)-and Di(alkylthio)-

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Electrochemical and Photophysical Properties. J. Org. Chem. 2007; 72: 2402-

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[12] Ohmi T. Manufacturing process of flat display. JSME Int J Ser B (Jpn Soc

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[13] Takamatsu T, Ogawa S, Ishii M. Color filter fabrication technology for

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[14] Kobayashi N. Design, synthesis, structure, and spectroscopic and

electrochemical properties of phthalocyanines. Bull. Chem. Soc. Jpn. 2002; 75:

1-19.

[15] Kim JH, Masaru M, Fukunishi K. Three dimensional molecular stacking

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bondings and interlayer π-π interactions. Dyes Pigm. 1998; 40: 53-57.

[16] Thetford D, Cherryman J, Chorlton AP, Docherty R. Theoretical molecular

modeling calculations on the solid state structure of some organic pigments.

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[17] Kim YD, Kim JP, Kwon OS, Cho IH. The synthesis and application of

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[18] Ma YS, Wang CH, Zhao YJ, Yu Y, Han CX, Qiu XJ, Shi Z. Perylene

diimide dyes aggregates: Optical properties and packing behavior in solution

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[19] Wϋrthner F, Sautter A, Thalacker C. Substituted Diazadibenzoperylenes:

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soluble resins for light-shielding black matrix application. Eur. Polym. J. 2009;

45: 474-484.

[21] Koo HS, Chen M and Kawai T. Improvements in the optical and

characteristics of black matrix films containing carbon nanotubes on color

filters. Diamond Relat. Mater. 2009; 18: 533-536.

[22] Yanık H, Aydın D, Durmuş M and Ahsen V. Peripheral and non-peripheral

tetrasubstituted aluminium, gallium and indium phthalocyanines: Synthesis,

photophysics and photochemistry. J. Photochem. Photobiol. A–Chem. 2009;

206: 18-26.

[23] M. Durmus and T. Nyokong, Inorg. The synthesis, fluorescence behaviour

and singlet oxygen studies of new water-soluble cationic gallium(III)

phthalocyanines. Chem. Commun. 2007; 10: 332-338.

[24] Li H, Jensen TJ, Fronczek FR and Vincente MGH. Syntheses and

Properties of a Series of Cationic Water-Soluble Phthalocyanines. J. Med. Chem.

2008; 51: 502-511.

[25] Tilley R. Colour and the optical properties of materials (1st edn). John

Wiley: New York, 2000; 30-31.

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[26] Zucchi G, Donnio B and Geerts YH. Remarkable miscibility between disk-

like and lathlike mesogen. Chem. Mat. 2005; 17: 4273-4277.

[27] Miller JD, Veeramasuneni S, Drelich J, Yalamanchili MR and Yamauchi G.

Effect of roughness as determined by atomic force microscopy on the wetting

properties of PTFE thin films. Polymer Eng Sci. 1996; 36: 1849-1855.

86

Chapter 4

The effect of five-membered heterocyclic bridges and

ethoxyphenyl substitution on the performance of phenoxazine-

based dye-sensitized solar cells

4.1 Introduction

Dye-sensitized solar cells (DSSCs) have attracted considerable attention as

promising solar devices since Grätzel et al. reported Ru-based photosensitizers

in 1991 [1]. The Ru complex dyes typical used as sensitizers in DSSCs have

shown high electronic conversion efficiencies of over 11% with good stability

[2]. However, high production cost and difficulties in purification have limited

their development for large-scale applications. Recently, more attention has

been paid to sensitizers without Ru (metal-free organic dyes and organometallic

dyes) due to their lower cost, easier modification and purification, high molar

extinction coefficient, and environmental friendliness. As such, sensitizers

without Ru such as triphenylamine [3], indoline [4], cyanine [5], coumarin [6],

perylene [7], porphyrin [8], phthalocyanine [9], and phenothiazine [10] have

87

been extensively studied. Among these, porphyrin derivatives have shown high

electronic conversion efficiency (12.3%) [7].

For efficient DSSCs, organic dyes should have broad and red-shifted

absorptions in the visible region. Accordingly, most organic dyes have the

structure of donor-conjugated bridges-acceptor (D-π-A) to obtain a broad and

red-shifted absorption spectrum. Among the various conjugated bridges, furan

and thiophene have displayed the most remarkable results, showing wide and

red-shifted absorption spectra, as well as high molar extinction coefficients [11].

In addition, to achieve enhanced photovoltaic performance, organic dyes with

an additional donor (D-D-A or D-D-π-A) have been suggested [12]. The

introduction of an additional donor group could increase the electron-donating

capability, which would improve electron injection and charge separation.

Phenoxazine (POZ) includes electron-rich oxygen and nitrogen atoms in a

heterocyclic ring, which displays high electron-donating ability [13]. It also

shows sufficient electrochemical properties, which implies that POZ could be a

promising sensitizer in DSSCs [14]. However, despite these advantages, POZ-

based sensitizers have not been reported extensively.

In this research, to study effects of conjugated bridges with a POZ moiety on

photovoltaic performance, five-membered heterocyclic rings were introduced as

88

a conjugated bridge unit to POZ molecules. The addition of these bridge units

could extend the conjugation of the dye molecule, which red-shifted the

absorption spectrum and increased the molar absorptivity of the dyes.

Furthermore, to improve the donating power and molar extinction coefficient,

an ethoxy phenyl ring was substituted in the 7 position of the POZ-furan dye as

an additional donor.

Based on these strategies, three organic dyes (WS1, WS2 and WS3) were

designed and synthesized. The photophysical and electrochemical properties of

the synthesized dyes were investigated in detail and density functional theory

(DFT) calculations were also performed. Photovoltaic cells were assembled

with the synthesized dyes and their photovoltaic properties were analyzed. In

addition, electrochemical impedance spectroscopy (EIS) was used to study the

interfacial charge transport process in the photovoltaic cells.

4.2 Experimental


89

Phenoxazine, N-bromosuccinimide and 4-ethoxyphenylboronic acid were

purchased from TCI and used as received without further purification. 1-

bromobutane, 5-formyl-2-furan-boronic acid, 5-formyl-2-thiophene-

boronicacid, tetrakis(triphenylphosphine) palladium(0), phosphorus oxychloride,

4-ethoxyphenylboronic acid, cyanoacetic acid and piperidine were purchased

from Sigma-Aldrich and used as received without further purification. All

solvents (chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide,

dichloromethane, 1, 2-dichloroethane and acetonitrile) were obtained from

Sigma-Aldrich and used as received. Other chemicals were reagent grade and

used without further purification.


1H NMR and 13C NMR spectra were recorded on a Bruker Avance 300, 500

and 600MHz using DMSO with the chemical shift against TMS (Seoul

National University National Center for Inter-University Research Facilities).

Mass data were measured using a JEOL JMS 600W mass spectrometer (Seoul

National University National Center for Inter-University Research Facilities).

UV-vis spectra were measured with a Hewlett-Packard 8425A

90

spectrophotometer. Cyclic voltammetry spectra were obtained using a three-

electrode cell with a 273A potentiostat (Princeton applied research, Inc.).

Measurements were performed using Ag wire (Ag/Ag+), glassy carbon and

platinum wire as the reference, working and counter electrodes, respectively, in

CH2Cl2 solution containing 0.1M tetrabutylammonium tetrafluoroborate

(TBATFB) as the supporting electrolyte. A standard ferrocene/ferrocenium

(Fc/Fc+) redox couple was employed to calibrate the oxidation peak.

Photocurrent-voltage measurements were performed using a Keithley model

2400 source measure unit. A 1000W Xe lamp (Spectra-physics) served as a

light source, and it was adjusted using an NREL-calibrated silicon solar cell

equipped with a KG-5 filter to approximate AM 1.5G sunlight intensity. The

incident photon-to-current conversion efficiency (IPCE) was measured as a

function of the wavelength from 300nm to 800nm using a specially designed

IPCE system for dye-sensitized solar cells (PV measurements, Inc.). A 75W Xe

lamp was employed as a light source to generate a monochromatic beam. The

electrical impedance spectra (EIS) of the DSSCs under dark with 0.60V

forward bias were measured with an impedance analyzer (Compactstat, IVIUM

Tech) at frequencies of 10-1 – 106 Hz. The magnitude of the alternative signal

91

was 10 mV. The impedance parameters were determined by fitting the

impedance spectra using Z-view software.


A Photoanode paste was prepared for a screen-printing process. The final

composition of the paste comprised TiO2 nanopowder (1 g), ethyl cellulose (0.5

g), terpineol (3.3 mL), and acetic acid (0.16 mL). After that, pre-washed FTO

glass was coated by a doctor blade process and then heated at 70℃ for 30

minutes for drying. After the printing, the TiO2 films were heated in four steps

of 325℃, 375℃, 450℃, and 500℃ for 5, 5, 15, and 15 minutes, respectively,

using a high-temperature furnace (Lab house Co.). For the post-treatment, the

coated and sintered TiO2 films were immersed in TiCl4 solution (40 mM in

water) for 30 min. at 70℃. After washing, the films were annealed at 500℃ for

30 minutes. Counter electrodes were prepared by spin coating method using 5

mM H2PtCl6 solution (in ethanol) on one-holed FTO glass and heated at 400℃

for 20 min. After cooling at 60℃, the TiO2 electrodes were immersed in

EtOH/CH2Cl2 solution containing the dyes at 0.5 mM for 48 h at ambient

temperature. After dye absorption, the photoanodes were washed using

92

anhydrous ethanol and dried under nitrogen flow. The dye-covered

photoelectrode and Pt-electrode were assembled using ionomer surlyn with a

hot-press at 80℃. After assembling, the electrolyte solution (composed of 0.6

M BMII, 0.05 M I2, 0.1 M LiI, and 0.5 M TBP in acetonitrile solvent) was

injected into the one-holed FTO glass using a capillarity vacuum technique, and

the hole was sealed with a cover-glass using the same surlyn. A black mask

aperture was placed on the front electrode for better analysis of the photovoltaic

characteristics. The active area of the dye-coated TiO2 film was ca. 0.24 cm2,

which was measured by analyzing the images from a CCD camera (moticam

1000). The TiO2 film thickness was measured by an α-step surface profiler

(KLA Tencor).

Photocurrent–voltage (I–V) measurements were performed using a Keithley

model 2400 source measure unit. A class-A solar simulator (Newport) equipped

with a 150 W Xe lamp was used as the light source. The light intensity was

adjusted with an NREL-calibrated Si solar cell with a KG-5 filter for

approximating the light intensity of 1 sun. Photocurrent–voltage measurements

of the dye-sensitized solar cells were performed with an aperture mask by

following a reported method. Incident photon-to-current conversion efficiency

(IPCE) was measured as a function of wavelength from 300 to 1000 nm using a

93

specially designed IPCE system for dye-sensitized solar cells (PV

measurements, Inc.). A 75 W xenon lamp was used as the light source for

generating monochromatic beams. Calibration was performed using a silicon

photodiode, which was calibrated based on the NIST-calibrated photodiode

G425 standard. The IPCE values were measured under halogen bias light at a

low chopping speed of 10 Hz. All calculations were carried out using Gaussian

09 software. Optimized geometries, energy levels, and frontier molecular

orbitals of the dyes’ HOMOs and LUMOs were calculated at the B3LYP/6-31G

(d,p) level.


4.2.4.1 10-Butyl-10H-phenoxazine (1)

Sodium hydroxide (7.36g, 0.184mol) and 1-bromobutane (6.62g, 0.048mol)

were slowly added to a phenoxazine (4.0g, 0.022mol) solution in dry DMSO

(50mL) at room temperature and stirred for 24h. Then, the reaction mixture was

poured into water and extracted with ethyl acetate. The organic phase was

separated and dried over anhydrous MgSO4. After removing the solvent, the

94

residue was purified by column chromatography using ethyl acetate-hexane

(1:10; v/v) as the eluent to give 1, colorless viscous liquid (4.78g, 91%).

1H NMR (500MHz, d6-DMSO) : δ = 6.81 (d, J = 8.7Hz, 2H), 6.63-6.67 (m,

4H), 3.53 (t, J = 7.7 Hz, 2H), 1.50-1.54 (m, 2H), 1.38-1.43 (m, 2H), 0.94 ppm (t,

J = 7.3 Hz, 3H).

4.2.4.2 10-Butyl-10H-phenoxazine-3-carbaldehyde (2)

POCl3 (0.55mL, 0.006mol) was added dropwise to a solution of 1 (1.29g,

0.005mol) and dry DMF (5mL) in dry1,2-dichloroethane (10.7mL) in an ice

water bath with temperature below 15℃. The reaction was heated to room

temperature and refluxed at 90℃ for 48h. The mixture was quenched with

dilute NaOH (aq) and extracted with water and dichloromethane (DCM). The

organic phase was dried with anhydrous MgSO4, and then the solvent was

removed in vacuo. The residue was purified by column chromatography using

ethyl acetate-hexane (1:6; v/v) to give 3, yellow oil (0.957g, 71.6%).

1H NMR (500MHz, d6-DMSO) : δ = 9.64 (s, 1H), 7.41 (dd, J = 8.3, 1.8 Hz,

1H), 7.00 (s, 1H), 6.68-6.87 (m, 5H), 3.62 (t, J = 7.9 Hz, 2H), 1.52-1.56 (m, 2H),

1.40-1.45 (m, 2H), 0.95 ppm (t, J = 7.3 Hz, 3H).

95

4.2.4.3 3-Bromo-10-butyl-10H-phenoxazine (3a)

1 (1.488g, 0.0062mol) and N-bromosuccinimide (1.106g, 0.0062mol) were

dissolved in chloroform (30mL), and the reaction was stirred for 1h at ambient

temperature. The reaction was quenched with water and extracted with water

and DCM. The organic phase was collected and the solvent was removed by

rotary evaporation. The residue was purified by column chromatography using

hexane to give 3a, white solid (1.48g, 75%).

1H NMR (500MHz, d6-DMSO) : δ = 7.0-6.96 (m, 1H), 6.86-6.80 (m, 2H),

6.69-6.59 (m, 4H), 3.52-3.47 (m, 2H), 1.53-1.46 (m, 2H), 1.44-1.35 (m, 2H),

0.90-0.91 ppm (m, 3H).

4.2.4.4 3,7-Dibromo-10-butyl-10H-phenoxazine (3b)

3b as a white solid (1.72g, 70%) was synthesized according to the procedure

described for the synthesis of 3a. N-bromosuccinimide (2.21g, 0.0124mol) was

added to a solution of 1 (1.49g, 0.0062mol) in chloroform (30mL) at ambient

temperature. Eluent : hexane.

1H NMR (500MHz, d6-DMSO) : δ = 7.0 (d, J = 8.6 Hz, 2H), 6.83 (s, 2H), 6.54

(d, J = 8.7 Hz, 1H), 3.50 (t, J = 7.5 Hz, 2H), 1.50-1.45 (m, 2H), 1.40-1.36 (m,

2H), 0.92 ppm (t, J = 7.3 Hz, 3H).

96

4.2.4.5 5-(10-Butyl-10H-phenoxazin-3-yl)furan-2-carbaldehyde (4a)

Under nitrogen atmosphere, a mixture of 3a (2.21g, 0.0069mol), 5-formyl-2-

furan-boronic acid (1.12g, 0.0080mol), 2M aqueous of K2CO3 (8.66mL),

Pd(PPh3)4 (0.4g, 0.00035mol) in dry THF (100mL) was stirred for 1/2 h and

refluxed at 80℃ overnight. The reaction was extracted with DCM, water and

brine. The organic phase was dried with anhydrous MgSO4, and then the

solvent was removed in vacuo. The residue was purified by column

chromatography using DCM-hexane (5:1; v/v) to give 4a, orange oil (1.13g,

49%).

1H NMR (500MHz, d6-DMSO) : δ = 9.53 (s, 1H), 7.60 (s, 1H), 7.33 (d, J = 8.4

Hz, 1H), 7.12 (s, 1H), 7.11 (s, 1H), 6.86 (t, J = 7.9 Hz, 1H), 6.78 (d, J = 8.5 Hz,

1H), 6.74-6.66 (m, 3H), 3.59 (t, J = 7.6 Hz, 2H), 1.57-1.52 (m, 2H), 1.45-1.40

(m, 2H), 0.95 ppm (t, J = 7.3 Hz, 3H).

4.2.4.6 5-(10-Butyl-10H-phenoxazin-3-yl)thiophene-2-carbaldehyde (4b)

4b as an orange oil (1.53g, 42%) was synthesized according to the procedure

described for the synthesis of 4a. 5-formyl-2-thiophene-boronic acid (1.95g,

0.0124mol) was added under nitrogen atmosphere to a solution of 3a (3.32g,

97

0.0104mol), 2M aqueous K2CO3 (13mL), Pd(PPh3)4 (0.61g, 0.00053mol) in dry

THF (100mL). Eluent : DCM-hexane (5:1; v/v).

1H NMR (300MHz, d6-DMSO) : δ = 9.84 (s, 1H), 7.96 (s, 1H), 7.59 (s, 1H),

7.26 (d, J = 8.3 Hz, 1H), 7.09 (s, 1H), 6.88-6.84 (m, 1H), 6.75-6.66 (m, 4H),

3.59 (t, J =7.9 Hz, 2H), 1.55-1.48 (m, 2H), 1.46-1.38 (m, 2H), 0.95 ppm (t, J =

7.2 Hz, 3H).

4.2.4.7 3-(5-Formyl-2-furan)-7-bromo-10-butyl-10H-phenoxazine (4c)

4c as an orange oil (1.03g, 41%) was synthesized according to the procedure

described for the synthesis of 4a. 5-formyl-2-furan-boronic acid (1.02g,

0.0072mol) was added under nitrogen atmosphere to a solution of 3b (2.41g,

0.0061mol), 2M aqueous K2CO3 (15.25mL), Pd(PPh3)4 (0.71g, 0.00061mol) in

dry THF (100mL). Eluent : DCM-hexane (5:1; v/v).


Hz, 1H), 7.13 (m, 2H), 7.02 (d, J = 8.4 Hz, 1H), 6.85 (s, 1H), 6.82 (d, J = 8.6 Hz,

1H), 6.68 (d, J = 8.7 Hz, 1H), 3.58 (t, J = 7.5 Hz, 2H), 1.54-1.50 (m, 2H), 1.43-

1.39 (m, 2H), 0.94 ppm (t, J = 7.3 Hz, 3H).

98

4.2.4.8 5-(3-(4-Ethoxyphenyl)-10-butyl-10H-phenothiazin-7-yl)furan-2-carbal

dehyde (5a)

A mixture of 4c (0.5g, 0.0012mol), 2M aqueous K2CO3 (6ml), Pd(PPh3)4

(0.07g, 0.00006mol) in dry THF (15mL) was refluxed for 1/2 h. 4-

ethoxyphenylboronic acid (0.28g, 0.0017mol) dissolved in dry THF (5mL) was

added to the reaction mixture and refluxed for 15h. The mixture was quenched

with water and extracted with DCM. The organic phase was dried with

anhydrous MgSO4, and then the solvent was removed in vacuo. The residue

was purified by column chromatography using DCM-ethyl acetate (10:1; v/v) to

give 5a, orange oil (0.29g, 53%).


Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H), 7.13-7.07 (m, 3H), 6.94 (d, J = 8.7 Hz, 2H),

6.91 (s, 1H), 6.77 (t, J = 8.7 Hz, 2H), 4.06-4.00 (m, 2H), 3.62 (t, J = 7.5 Hz, 2H),

1.58-1.56 (m, 2H), 1.45-1.43 (m, 2H), 1.34 (t, J = 7 Hz, 3H), 0.97 ppm (t, J =

7.3 Hz, 3H).

4.2.4.9 (E)-(10-Butyl-10H-phenoxazin-3-yl)-2-cyanoacrylic acid (POX)

3 (0.23g, 0.00086mol), cyanoacetic acid (0.22g, 0.0026mol) and piperidine

(0.18mL, 0.00347mol) were added to anhydrous CH3CN (100mL). After the

99

mixture was refluxed for 8h, the solution was extracted with DCM and 0.1M

HCl aqueous solution. The organic phase was dried over anhydrous MgSO4,

and the solvent was removed in vacuo The crude product was purified by

column chromatography using DCM- methanol (5:1; v/v) to give POX, red

solid (0.22g, 75%).

1H NMR (600MHz, d6-DMSO) : δ = 7.97 (s, 1H), 7.48 (d, J = 8.5 Hz, 1H),

7.36 (s, 1H), 6.85 (t, J = 7.6 Hz, 1H), 6.69-6.78 (m, 4H), 3.59 (t, J = 7.6 Hz, 2H),

1.50-1.55 (m, 2H), 1.37-1.43 (m, 2H), 0.93 ppm (t, J = 7.3 Hz, 3H) ;

13C NMR (150MHz, d6-DMSO) : δ = 164.1, 152.2, 143.8, 143.6, 137.8, 130.9,

130.6, 124.3, 123.7, 122.6, 117.2, 115.3, 114.5, 112.9, 111.7, 98.0, 42.9, 26.7,

19.2, 13.7 ppm ;

m/z (FAB) 334.1316 ((M +), C20H18N2O3 requires 334.1317).

4.2.4.10 (E)-3-(5-(10-Butyl-10H-phenoxazin-3-yl)furan-2yl)-2-cyanoacrylic

acid (WS1)

WS1 (0.13g, 54%) was synthesized according to the procedure described for

the synthesis of POX. 4a as a dark red solid (0.2g, 0.0006mol), cyanoacetic

acid (0.15g, 0.0018mol), and piperidine (0.24mL, 0.0024mol) were added to

anhydrous CH3CN (50mL). Eluent : DCM-methanol (9:1; v/v).

100

1H NMR (500MHz, d6-DMSO) : δ = 7.97 (s, 1H), 7.48 (s, 1H), 7.41 (d, J = 8

Hz, 1H), 7.19-7.17 (m, 2H), 6.87-6.84 (m, 1H), 6.81 (d, J = 9 Hz, 1H), 6.74-

6.72 (m, 1H), 6.70-6.68 (m, 2H), 3.60 (t, J = 7.5 Hz, 2H), 1.58-1.52 (m, 2H),

1.46-1.39 (m, 2H), 0.95 ppm (t, J = 7.5 Hz, 3H) ;

13C NMR (125MHz, d6-DMSO) : δ = 163.5, 158.0, 146.4, 143.7, 143.2, 136.4,

133.9, 131.1, 126.2, 123.7, 121.1, 121.0, 120.4, 116.4, 114.6, 111.9, 111.8,

110.6, 108.0, 42.6, 26.1, 18.7, 13.2 ppm ;


4.2.4.11 (E)-3-(5-(10-Butyl-10H-phenoxazin-3-yl)thiophene-2yl)-2-cyano

acrylic acid (WS2)

WS2 (0.43g, 79%) as a dark red solid was synthesized according to the

procedure described for the synthesis of POX. 4b (0.46g, 0.0013mol),

cyanoacetic acid (0.33g, 0.0039mol), and piperidine (0.51mL, 0.0052mol) were

added to anhydrous CH3CN (100mL). Eluent : DCM-methanol (10:1; v/v).

1H NMR (500MHz, d6-DMSO) : δ = 8.43 (s, 1H), 7.96 (s, 1H), 7.63 (s, 1H),

7.25 (d, J = 8.5 Hz, 1H), 7.08 (s, 1H), 6.87 (t, J = 7.5 Hz, 1H), 6.77-6.69 (m,

4H), 3.61 (t, J = 6.5 Hz, 2H), 1.59-1.53 (m, 2H), 1.47-1.40 (m, 2H), 0.96 ppm (t,

J = 7 Hz, 3H) ;

101

13C NMR (75MHz, d6-DMSO) : δ = 163.2, 151.9, 145.8, 143.8, 143.1, 141.0,

133.8, 132.6,131.3, 124.2, 123.8, 123.2, 122.1, 121.0, 116.2, 114.6, 111.9,

111.6, 66.4, 26.0, 18.8, 13.2 ppm ;

m/z (FAB) 417.1281 ((M + H +), C24H21N2O3 S requires 417.1273).

4.2.4.12(E)-3-(5-(4-Methoxyphenyl)-10-butyl-10H-phenoxazin-7-yl)furan-2-

yl)-2 cyanoacrylic acid (WS3)

WS3 (0.25g, 72%) as a dark purple solid was synthesized according to the

procedure described for the synthesis of POX. 5a (0.3g, 0.00066mol),

cyanoacetic acid (0.17g, 0.002mol) and piperidine (0.26mL, 0.0026mol) were

added to anhydrous CH3CN (100mL). Eluent : DCM-methanol (5:1; v/v).


7.49 (s, 1H), 7.44(d, J = 8.5, 1H), 7.22-7.20 (m, 2H), 7.13 (d, J = 8 Hz, 1H),

6.95 (m, 3H), 6.86 (d, J = 8.5 Hz, 1H), 6.80 (d, J = 8.5 Hz, 1H), 4.06-4.05 (m,

2H), 3.65 (t, J = 7 Hz, 2H), 1.59-1.57 (m, 2H), 1.48-1.43 (m, 2H), 1.34(d, J = 7

Hz, 3H), 0.97 ppm (t, J = 7 Hz, 3H) ;

13C NMR (75MHz, d6-DMSO) : δ = 163.5, 158.1, 157.3, 146.4, 143.6, 143.5,

136.5, 133.7, 132.8, 130.5, 129.8, 126.3, 121.3, 121.0, 120.4, 120.4, 116.4,

114.2, 112.3, 112.1, 111.9, 110.6, 108.2, 62.5, 42.2, 26.2, 18.8, 14.1, 13.3 ppm ;

102



4.3.1. Synthesis

The synthetic routes of POZ dyes are shown in Scheme 4.1 and the

synthesized dyes and reference dyes (POX) were shown in Fig. 4.1. The POZ

syntheses first involved an alkylation reaction on POZ nitrogen atom and a

butyl chain was introduced to the POZ moiety to give intermediate 1. The

bromination of 1 was performed by varying the amount of N-bromosuccinimde

(NBS) to provide mono-brominated 3a and dibrominated 3b, respectively. The

Suzuki coupling reactions of 3a were carried out with 5-formyl-2-furanboronic

acid or 5-formyl-2-thiopheneboronic acid, which produced 4a and 4b,

respectively. The Suzuki coupling reaction of 3b to give 4c followed the

procedure performed with 3a, and subsequently, an additional Suzuki coupling

reaction of 4c with 4-ethoxyphenylboronic acid gave 5a. The final compounds

(WS1, WS2, and WS3) were obtained by the Knoevaenagal reactions of the

corresponding aldehydes (4a, 4b, and 5a) with cyanoacetic acid in the presence

103

of piperidine [15]. The reference dye (POX) was obtained by previously

reported synthetic routes. The structures of all the synthesized intermediates

and dyes were identified by 1H NMR, and the final products were additionally

confirmed by 13C NMR and HRMS.

Scheme 4.1. Synthesis of POX, WS1, WS2, and WS3: (a) 1-iodobutane, NaOH,

DMSO (b) NBS, CHCl3 (c) POCl3, DMF, CHCl3 (d) 2M aqueous of K2CO3,

Pd(pph3)4, THF (e) cyanoacetic acid, piperidine, acetonitrile.

104

Fig. 4.1. Structure of POX, WS1, WS2, and WS3.

4.3.2. Photophysical properties

The absorption spectra of the dyes in EtOH/CH2Cl2 solution and on the TiO2

surface are shown in Fig. 4.2, and the corresponding photophysical data are

listed in Table 4.1. All the dyes exhibited two major absorption bands, which

appeared at below 350nm and 400-550nm, respectively. The former band in the

UV region is ascribed to a localized aromatic π-π* transition, and the latter band

105

in the visible region is due to the intramolecular charge-transfer (ICT) transition

from the donor to the acceptor. The absorption maxima (λmax) of WS1, WS2,

and WS3 are 452, 455, and 475 nm, respectively. The absorption spectrum of

WS3, which included both the additional donor and bridge moiety, was red-

shifted more than 10nm compared to those of WS1 and WS2, which included

the bridge unit only. This was attributed to the better delocalization of electrons

over the π-conjugated molecules by both the electron-rich furan and the ethoxy

phenyl ring. The molar extinction coefficient at λmax of WS1, WS2, and WS3

were 19063, 19209, and 26335 M-1cm-1, respectively. These are higher than

those of structurally similar phenothiazine dyes (PT 5, 17600 M-1cm-1) as well

as standard ruthenium dye (N719, 14000M-1cm-1), which afford the use of

thinner TiO2 film for efficient electron diffusion. Among these dyes, WS3

exhibited the highest molar extinction coefficient. This result indicated that the

ethoxy phenyl ring is beneficial to the conjugation and absorption properties of

WS3, despite the large dihedral angle between the POZ core and ethoxy phenyl

ring (Table 4.2). The red-shifts in the absorption spectra and high molar

extinction coefficients of the dyes have a tendency to increase with

enhancement of the electron density in the bridge unit and additional donor.

106

Thus, their introduction to the POZ moiety is favorable for enhancing the light

harvest and photocurrent generation of the dyes.

The absorption spectra of the dyes absorbed on TiO2 film are blue-shifted

compared to those in solution, and there is a larger blue-shift in WS3 (9nm)

than in WS1 and WS2 (3nm). This might be due to the H-aggregation of the

dyes or the deprotonation of carboxylic acid upon the adsorption onto the TiO2

surface [16]. WS3 showed almost the same absorbance on the TiO2 surface

compared to WS1 and WS2, although it exhibited a much higher molar

extinction coefficient in solution. As shown in Table 4.2 and Table 4.3, this is

due to the non-planar structure of WS3 caused by the introduction of the ethoxy

phenyl ring, which inhibits the adsorption of the dye on the TiO2 surface. The

optimized geometry of the dye in the ground state revealed that the POZ core of

all the dyes had planar structures with small torsion angles (0.82-1.07°). On the

other hand, there were considerable differences in the dihedral angles between

the POZ core and the substituents. WS1 had almost planar structure with a

small dihedral angle (0.29°) between the core of the dye and the furan moiety,

while, the planarity of WS2 and WS3 decreased significantly due to the large

dihedral angle between the POZ core and the adjacent bridge unit or donor

moiety; the dihedral angle between thiophene and the POZ core in WS2 was

107

19.36°, and that between the ethoxy phenyl ring and the POZ core in WS3 was

37.35°. WS3 had the most twisted molecular geometry of the dyes, which led to

less adsorption and decreased absorbance of the dye on the TiO2 surface (Table

4.3).

400 500 600 7000.00

0.05

0.10

0.15

0.20

0.25

0.30

(a)

Ab

sorb

an

ce (

AU

)

Wavelength (nm)

WS1 WS2 WS3

400 500 600 7000

1

2

3

4

(b)

Abs

orba

nce

(AU

)

Wavelength (nm)

WS1 WS2 WS3

Fig. 4.2. Absorbtion spectra of WS1, WS2, and WS3 in (a) EtOH/CH2Cl2 (7 : 2;

v/v) (10-5molL-1) and (b) on TiO2.

108

Table 4.1. Photophysical and electrochemical properties of WS1, WS2 and

WS3 dyes.

Dye

Absorptiona Oxidation potential datac

λmax/nm

ε/M-1c m-1

(at λmax)

λabsb/nm

(on TiO2)

Eox/V

(vs. NHE)

E0-0d/V

Eox - E0-0/V

(vs. NHE)

WS1 452 19063 449 0.88 2.36 -1.48 WS2 455 19209 452 0.89 2.33 -1.44

WS3 475 26335 466 0.81 2.13 -1.31

a Measured in 1x 10-5 EtOH/CH2Cl2 solutions at room temperature.

b Measured on TiO2 film. c Measured in CH2Cl2 containing 0.1 M tetrabutylammonium tetrafluoroborate. (TBABF4) electrolyte (working electrode: glassy carbon; counter electrode: Pt; reference electrode: Ag/Ag+; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV.)

d Estimated from onset wavelength in absorption spectra

109

Table 4.2. Optimized structures, dihedral angles and electronic distributions in


Dye Optimized structure HOMO LUMO

WS1

WS2

WS3

110

Table 4.3. DSSC performance parameters of POX, WS1, WS2, and WS3a.

Dyeb Jsc

(mA/cm2)

Voc

(mV)

FF

(%)

η

(%)

Dye amountc

(10–7mol cm–2)

POX 9.88 663.5 73.22 4.80 -

WS1 11.72 653.0 68.76 5.26 2.05

WS2 11.11 628.3 70.38 4.91 1.89

WS3 11.35 639.9 67.09 4.87 1.34

N719 14.71 728.3 72.85 7.80 -

a Measured under AM 1.5 irradiation G (100 mW cm–1); 0.2 cm2 working area. b Dyes were

maintained at 0.5 mM in EtOH/CH2Cl2 solution, with 10 mM CDCA co-adsorbent. Electrolyte

comprised 0.7 M 1-propryl-3-methyl-imidazolium iodide (PMII), 0.2 M LiI, 0.05 M I2, 0.5 M

TBP in acetonitrile-valeronitrile (v/v, 85/15) for organic dyes. c Dyes' adsorption on TiO2 were

measured by a colorimetric method using 0.1 M NaOH aqueous-DMF (1:1) mixed solutions to

wash the dyes from 8 μm thick TiO2 film.

4.3.3. Electrochemical properties

The electrochemical properties and energy levels of the prepared dyes are

provided in Table 4.1 and Fig. 4.3. The oxidation potentials of the dyes were

111

measured using cyclic voltammetry (CV), and the C-V curves of the dyes are

shown in Fig. 4.4. The HOMO levels of the prepared dyes correspond to the

first oxidation potential versus a normal hydrogen electrode (vs. NHE)

calibrated by Fc/Fc+ (with 640 mV vs. NHE). All of the HOMO levels of the

dyes ranged from 0.81 eV to 0.88 eV (vs. NHE) and were sufficiently positive

compared to the redox potential of I-/I3- (0.4 V vs. NHE), implying that the

oxidized dyes can be effectively regenerated by the redox electrolyte [17]. The

LUMO levels of the dyes were obtained by subtracting the zeroth-zeroth energy

(E0-0) from Eox, in which the zeroth-zeroth energy of the dyes is determined

from the inflection point at the end of the visible absorption spectrum of the

dyes. All of the LUMO levels of the dyes were more negative than the

conduction band energy level (Ecb) of TiO2 (-0.5 vs. NHE), indicating that the

excited electrons of the dyes can be efficiently injected into the conduction

band of TiO2, and oxidized dyes are simultaneously formed [17]. As shown in

Table 4.1, WS3 had a more negative HOMO level and a more positive LUMO

level than WS1 and WS2. This might be attributed to the extension of

conjugation induced by the additional donating group. Consequently, the gap

between the HOMO and LUMO decreased, and the absorption spectra became

red-shifted.

112

Fig. 4.3. Dyes’ HOMO and LUMO energy levels.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8-0.000006

-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006

0.000008

0.000010

0.000012

Cu

rre

nt (

J)

Voltage (V)

Fer POX WS1 WS2 WS3

Fig. 4.4. CV curves of Fc/Fc+, POX, WS1, WS2, and WS3 in CH2Cl2.

113

4.3.4. Photovoltaic properties

DSSCs were fabricated using the dyes as sensitizers, and their photovoltaic

properties were measured under the standard AM 1.5G irradiation conditions

(100mW cm-2). The effects of the five-membered heterocyclic bridges and the

ethoxyphenyl substitution on the performance of the cells were evaluated by

measuring their conversion efficiency relative to the reference dye (POX). The

IPCE spectra and photocurrent-voltage (J-V) curves are shown in Fig. 4.5, and

the corresponding data are listed in Table 4.3. All the dyes showed high

maximum IPCE values at 400-500nm, which were 73.4%, 69.7%, and 73.2%

for WS1, WS2, and WS3, respectively. These values were higher than that of

standard ruthenium dye (N719, 69.6%). Therefore, all the dyes can effectively

convert visible light into photocurrent in the absorption ranges. The IPCE

spectra of WS1 and WS2 showed broadened and red-shifted ranges compared

to that of POX, and the onsets of their IPCE spectra were extended to 750nm.

The bridge units introduced to the POZ moiety red-shifted the spectra of the

dyes due to the extension of conjugation, increasing light harvest in the long

wavelength region. WS3, which had an additional donor, showed a similar

114

IPCE spectrum to that of WS1. This might be attributed to the fact that the

absorption spectrum of WS3 on TiO2 was largely blue-shifted compared to that

of WS1. In addition, it is known that a larger amount of dye adsorbed on TiO2

leads to a broader IPCE spectrum [18]. Thus, the lesser amount of adsorption of

WS3 on the TiO2 surface also contributed the relatively narrow IPCE spectrum

of the dye. The short-circuit current (Jsc) values increased in the order of POX <

WS2 < WS3 < WS1, and this trend correlated with the broadness of the IPCE

spectrum. Consequently, the introduced bridge units improved Jsc and the

conversion efficiency, while the introduction of the additional donor group to

the POZ moiety had little effect on Jsc.

The open-circuit voltage (Voc) values of the cells fabricated with the dyes

increased in the order of WS2 < WS3 < WS1 < POX. The Voc values of the

cells based on WS1-3 are lower than that based on POX. As shown in previous

studies, this might be explained by the increased interaction caused by the

introduction of a heterocyclic bridge unit from π- π interaction between the dye

molecules and/or the interaction between the heteroatom in the bridge unit and

iodine [19, 11c]. These interactions enhanced the charge recombination at the

dye/dye or dye/electrolyte interface, resulting in low Voc values of the cells. As

shown in Fig. 4.6, this tendency was also shown in the dark currents of the cells

115

made with the dyes. The dark current is defined as the relatively small electric

current flowing out of a system without illumination and lower dark current

indicates decreased recombination and high Voc. Thus, the low dark current of

the cell with POX corresponds with the highest Voc, whereas the high dark

current of the cell with WS2 is consistent with the lowest Voc.

To explain the correlation between the Voc of the cells and the dyes,

electrochemical impedance spectroscopy (EIS) [20] was carried out in the dark

under a forward bias of -0.60 V. The Nyquist plots of the DSSCs fabricated

with the dyes are shown in Fig. 4.7. In the Nyquist plot, the major semicircle at

the intermediate frequency represents the charge transfer impedance at the

TiO2/dye/electrolyte interface. The charge recombination resistance can be

estimated by the radius of the major semicircle at the intermediate frequency,

with a larger radius of the major semicircle meaning a smaller charge

recombination rate [21]. The charge recombination resistance increased in the

order of WS2 < WS3 < WS1 < POX, which is in accordance with the Voc

values in the DSSCs made with the dyes. As the charge recombination between

injected electrons and the electrolyte increased, Voc increased as well. As shown

in Fig. 4.8, this result coincides with the electron lifetime vs dark bias voltage.

The electron lifetime increased in the order of WS2 < WS3 ≈ WS1 < POX <

116

N719. Longer electron lifetime indicates improved charge recombination

resistance between the injected electrons and electrolyte, with a consequent

increase in the Voc [22]. The results suggest that the introduction of the furan

bridge unit retards the charge recombination more effectively compared to the

thiophene bridge unit, leading to a longer electron lifetime and a higher Voc.

400 500 600 700 8000

10

20

30

40

50

60

70

80 (a)

IPC

E (

%)

Wavelength (nm)

N719 POX WS1 WS2 WS3

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

2

4

6

8

10

12

14

16

18

20

(b)

N719POXWS1WS2WS3

Cur

rent

de

nsity

(m

A/c

m2 )

Voltage (V)

Fig. 4.5. (a) IPCE spectra DSSCs based on POX, WS1, WS2, WS3, and N719

and (b) the DSSCs' J–V curves under AM 1.5G simulated sunlight (100 mWcm-

2).

117

-0.2 0.0 0.2 0.4 0.6 0.8-20

-15

-10

-5

0

5

POXWS1WS2WS3

Cur

rent

de

nsity

(m

A/c

m2)

Voltage (V)

Fig. 4.6. DSSCs' J–V curves based on POX, WS1, WS2, and WS3 in the dark.

0 5 10 15 20 25 300

-5

-10

-15

-20

-25

Z"

(Oh

m)

Z' (Ohm)

POX WS1 WS2 WS3

Fig. 4.7. Impedance spectra of DSSCs based on POX, WS1, WS2, and WS3;

Nyquist plots measured at 0.60 V forward bias in the dark.

118

0.50 0.55 0.60 0.65 0.70 0.7510

100

N719 POX WS1 WS2 WS3

Life

tim

e (m

s)

Bias voltage (V)

Fig. 4.8. Electron lifetime of N719, POX, WS1, WS2, and WS3 as a function

of bias voltage.

4.4 Conclusion

The first design and synthesis of three novel phenoxazine-based organic dyes

(WS1, WS2 and WS3) has been done to study the effects of the various bridge

units and an additional donor on the performance of DSSCs. The introduction

of the heterocyclic bridge units (furan and thiophene) extended the conjugation

119

and red-shifted the absorption spectra of the dyes, improving Jsc. Consequently,

the DSSCs based on WS1 with the furan unit and WS2 with the thiophene unit

showed higher overall conversion efficiencies in comparison with the reference

dye (POX) without the bridge unit. The most effective bridge unit was furan,

which had a higher recombination resistance and Voc value. WS3 with the

ethoxy phenyl ring as an additional donor showed more red-shift in the

absorption, whereas the planarity of the dye was reduced by the large dihedral

angle between the ethoxy phenyl ring and the POZ core. This non-planar

structure of the dye led to less adsorption of the dye on the TiO2 surface, which

limited the Jsc enhancement. The DSSC based on WS1 with the furan bridge

unit exhibited the highest overall conversion efficiency of 5.26% (Jsc = 11.72

mA/cm2, Voc = 653 mV, FF = 68.76).

4.5 References

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127

Chapter 5

The effects of the number of anchoring groups and N-

substitution on the performance of phenoxazine dyes in dye-

sensitized solar cells

5.1 Introduction

The demand for environmentally friendly energy sources continues to

increase due to depletion of traditional energy sources and due to environmental

consideration. One of the attractive candidates of these new energy sources is

solar energy, which is clean, renewable and limitless. Dye-sensitized solar cells

(DSSCs) have been considered as one of the promising energy harvesting

devices since the report of Ru-based photosensitizers in 1991 by Grätzel et al

[1]. The Ru complex sensitizers (N3, N719 and black dye) have shown high

photoelectric conversion efficiencies of over 11% under AM 1.5 conditions [2].

Compared with them, metal-free organic dyes and organometallic dyes have

advantages including lower cost, easier modification and purification,

environmental friendliness and high molar extinction coefficient. The highest

conversion efficiencies of organic dyes (10% [3]) and organometallic dyes

128

(12.3% [4]) demonstrated that non-Ru dyes could be promising sensitizers for

realizing highly efficient DSSCs. For these reasons, sensitizers containing

coumarin [5], carbazole [6], fluorene [7], hemicyanine [8], indoline [9],

merocyanine [10], perylene [11], polyene [12], porphyrin [4,13],

phthalocyanine [14], triphenylamine [3,15] have been extensively studied.

Phenoxazine (POZ)-based sensitizers have exhibited higher conversion

efficiencies than triphenylamine (TPA) and phenothiazine (PTZ)-based

sensitizers, which are structurally similar [16,17,18]. This is because POZ-

based sensitizers, with electron-rich nitrogen and oxygen heteroatoms, have

stronger electron-donating ability than TPA and PTZ-based sensitizers. POZ-

based sensitizers also show sufficient electrochemical properties for use in

DSSCs [19]. However, despite their potential for application to DSSCs, POZ-

based sensitizers have not been studied extensively.

We report the introduction of an additional cyanoacrylic acid moiety in the 7-

position of the POZ chromophore as the second anchoring group. Compared to

mono-anchoring sensitizers, this di-anchoring sensitizer has increased electron

pathways and extended conjugations of the POZ moiety. Therefore, the short-

circuit current (Jsc) can be improved by the bathochromic shift of the absorption

spectrum. However, the di-anchoring dyes have exhibited lower open-circuit

129

voltages (Voc) compared to the mono-anchoring dyes. Therefore, to improve

Voc of cells based on di-anchoring dyes, a bulky methoxyphenyl ring was

introduced to the POZ nitrogen atom. The steric hinderance by these bulky

groups was expected to reduce dye aggregation and suppress the electron

recombination between the electrons injected on the TiO2 and the holes in the

electrolyte to result in high Voc.

Four POZ derivatives were designed and synthesized, i.e., POX, WB, WH1

and WH2 as shown in Fig. 5.1. To examine the effects of the additional

anchoring group and the N-substituents on the performance of DSSCs,

photophysical and electrochemical properties of the dyes and photovoltaic

performance of the cells based on these dyes were analyzed. In addition,

electrochemical impedance spectroscopy (EIS) was used to investigate

interfacial charge transport process. Density functional theory (DFT)

calculations were also performed for further analysis of the results.

130

O

NCN

COOH O

NCN

HOOC

CN

COOH

O

NCN

HOOC

CN

COOH

O

O

NCN

COOH

O

POX WB

WH1 WH2

Fig. 5.1. Structure of POX, WB, WH1 and WH2.

131

5.2 Experimental


Phenoxazine, 1-bromobutane, 4-iodoanisole, copper-tin alloy, 18-crown-6,

phosphorus oxychloride, cyanoacetic acid and piperidine were purchased from

Sigma-Aldrich and used as received without further purification. All solvents

(dimethylformamide, 1,2-dichlorobenznene, dimethyl sulfoxide,

dichloromethane, 1,2-dichloroethane and acetonitrile) were obtained from

Sigma-Aldrich and used as received. Other chemicals were reagent grade and

used without further purification.


1H NMR and 13C NMR spectra were recorded on a Bruker Advance 500 and

600MHz (Seoul National University National Center for inter-University

Research Facilities) with the chemical shift against TMS. Mass data were

measured with a JEOL JMS 600W mass spectrometer (Seoul National

University National Center for inter-University Research Facilities). ATR-FTIR

132

spectra were recorded on a Nicolet 6700 spectrometer by using the window

ZnSe/diamond ATR accessory. UV-vis spectra and photoluminescence spectra

were recorded on a Hewlett-Packard 8425A spectrophotometer and a Shimadzu

RF-5301PC specrofluorometer, respectively. Cycilc voltammetry spectra were

obtained using a three-electrode cell with a 273A potentiostat (Princeton

applied research, Inc.). Measurements were taken using a Ag wire (Ag/Ag+), a

glassy carbon and a platinum wire as the reference, working and counter

electrodes, respectively, in the DMF solution containing 0.1M

tetrabutylammonium tetrafluoroborate (TBATFB) as the supporting electrolyte.

A standard ferrocene/ferrocenium (Fc/Fc+) redox couple was used to calibrate

the oxidation peak. Photocurrent-voltage measurements were performed using a

Keithly model 2400 source measure unit. Incident photon-to-current conversion

efficiency (IPCE) was measured as a function of wavelength from 300nm to

1000nm using a specially designed IPCE system for dye-sensitized solar cells

(PV measurements, Inc.). Electrical impedance spectra (EIS) of DSSCs under

dark with 0.55 V forward bias and under illumination at an open-circuit voltage

were measured with an impedance analyzer (Compactstat, IVIUM Tech) at

frequencies of 10-1 – 106 Hz. The magnitude of the alternative signal was 10

133

mV. Impedance parameters were determined by fitting the impedance spectra

using the Z-view software.


The nanocrystalline TiO2 working electrode was comprised of a TiO2

transparent layer (20 nm, synthesized) and a TiO2 scattering layer (250 nm, G1).

The Pt-coated counter electrode was prepared by a reported procedure [20].

TiO2 electrodes were immersed in THF solution containing the dyes at 0.5 mM

for 40 h at ambient temperature. They were then washed with ethanol and dried

under a stream of nitrogen. The working and counter electrodes were sealed

with Surlyn (60μm, Dupont) and electrolyte was injected through a hole in the

counter electrode. The electrolyte was comprised of 0.7 M 1-propryl-3-methyl-

imidazolium iodide (PMII, synthesized), 0.2 M LiI (Aldrich), 0.05 M I2

(Aldrich), and 0.5 M 4-tert-butylpyridine (Aldrich) in a mixed solvent of

acetonitrile and valeronitrile (v/v, 85/18). The active area of the dye-coated

TiO2 film was ca. 0.24 cm2, measured by analyzing the images from a CCD

camera (moticam 1000). TiO2 film thickness was measured by an α-step surface

profiler (KLA tencor).

134

Photocurrent–voltage (I–V) measurements were performed using a Keithley

model 2400 source measure unit. A class-A solar simulator (Newport) equipped

with a 150 W Xe lamp was used as the light source. Light intensity was

adjusted with an NREL-calibrated Si solar cell with KG-5 filter for

approximating 1 sunlight intensity. Photocurrent–voltage measurements of the

dye-sensitized solar cells were performed with an aperture mask following a

reported method. Incident photon-to-current conversion efficiency (IPCE) was

measured as a function of wavelength from 300 to 1000 nm using a specially

designed IPCE system for dye-sensitized solar cells (PV measurements, Inc.). A

75 W xenon lamp was used as the light source for generating monochromatic

beams. Calibration was performed using a silicon photodiode, which was

calibrated based on the NIST-calibrated photodiode G425 standard. IPCE

values were measured under halogen bias light at a low chopping speed of

10 Hz. All calculations were carried out using the Gaussian 09 software.

Optimized geometries, energy levels, and frontier molecular orbitals of the dyes’

HOMOs and LUMOs were calculated at the B3LYP/6-31G (d,p) level.

135


5.2.4.1 10-Butyl-10H-phenoxazine (1)

To a phenoxazine (1.5g, 0.0082mol) solution in dry DMSO (22.5mL), sodium

hydroxide (2.76g, 0.069mol) and 1-bromobutane (1.89g, 0.0137mol) were

slowly added at room temperature and stirred for 24h. Then the reaction

mixture was poured into water and extracted with ethyl acetate. The organic

phase was separated and dried over anhydrous MgSO4. After removing the

solvent, the residue was purified by column chromatography using ethyl

acetate-hexane (1:10; v/v) as the eluent to give 1, colorless viscous liquid (1.8g,

92%). 1H NMR (500MHz, d6-DMSO) : δ = 6.81 (d, J = 8.7Hz, 2H), 6.63-6.67

(m, 4H), 3.53 (t, J = 7.7 Hz, 2H), 1.50-1.54 (m, 2H), 1.38-1.43 (m, 2H), 0.94

ppm (t, J = 7.3 Hz, 3H).

5.2.4.2 10-(4-Methoxyphenyl)-10H-phenoxazine (2)

Under nitrogen atmosphere, phenoxazine (1.5g, 0.0082mol), 4-iodoanisole

(2.88g, 0.0123mol), CuSn (1.49g, 0.0082mol), K2CO3 (3.4g, 0.0246mol) and

18-crown-6 (0.38g, 0.00145mol) were dissolved in dry 1,2-dichlorobenznene

(60mL). The mixture was heated under refluxed for 48h under nitrogen

136

atmosphere. Then the reaction mixture was filtered and washed with

dichloromethane (DCM). The filtrate was extracted with DCM, water and

NH4OH. The organic phase was collected and dried over anhydrous MgSO4.

After removing solvent, the residue was purified by column chromatography

using DCM-hexane (1:3; v/v) to give 2, viscous pale yellow liquid (1.42g,

81.6%).

1H NMR (300MHz, d6-DMSO) : δ = 7.30 (d, J = 8.6 Hz, 2H), 7.18 (d, J = 8.6

Hz, 2H), 6.62-6.72 (m, 6H), 5.85 (d, J = 9.2 Hz, 2H), 3.83 ppm (s, 3H).

5.2.4.3 10-Butyl-10H-phenoxazine-3-carbaldehyde (3)

To a solution of 1 (2.57g, 0.01mol) and dry DMF (5mL) in dry1,2-

dichloroethane (21.4mL) in an ice water bath, POCl3 (1.1mL, 0.012mol) was

added dropwise below 15℃. The reaction was heated to room temperature and

heated under refluxed at 90℃ for 48h. The mixture was quenched with dilute

NaOH (aq) and extracted with water and DCM. The organic phase was dried

with anhydrous MgSO4 and then the solvent was removed in vacuo. The

residue was purified by column chromatography using ethyl acetate-hexane

(1:6; v/v) to give 3, yellow oil (1.91g, 71.6%).

137


1H), 7.00 (s, 1H), 6.68-6.87 (m, 5H), 3.62 (t, J = 7.9 Hz, 2H), 1.52-1.56 (m, 2H),

1.40-1.45 (m, 2H), 0.95 ppm (t, J = 7.3 Hz, 3H).

5.2.4.4 10-Butyl-10H-phenoxazine-3,7-dicarbaldehyde (4)

4 as an orange solid (1.46g, 49.5%) was synthesized according to the

procedure described above for the synthesis of 3. To a solution of 1 (2.57g,

0.01mol) and dry DMF (5mL) in dry 1,2-dichloroethane (21.4mL) in an ice

water bath, POCl3 (9.17mL, 0.1mol) was added dropwise below 15℃. Eluent :

DCM- methanol (7:1; v/v).


2H), 7.05 (s, 2H), 6.94 (d, J = 8.4 Hz, 2H), ), 3.68 (t, J = 7.7 Hz, 2H), 1.52-1.57

(m, 2H), 1.41-1.45 (m, 2H), 0.95 ppm (t, J = 7.3 Hz, 3H).

5.2.4.5 10-(4-Methoxyphenyl)-10H-phenoxazine-3-carbaldehyde (5)

5 as a yellow solid (0.41g, 76.2%) was synthesized according to the procedure

described above for the synthesis of 3. To a solution of 2 (0.48g, 0.0017mol)

and dry DMF (0.39mL) in dry1,2-dichloroethane (21.4mL) in an ice water bath,

POCl3 (1.1mL, 0.012mol) was added dropwise below 15℃. Eluent : DCM.

138

1H NMR (500MHz, d6-DMSO) : δ = 9.62 (s, 1H), 7.36 (d, J = 9 Hz, 2H), 7.21-

7.25 (m, 3H), 7.09 (s, 1H), 6.67-6.77(m, 3H), 5.96 (d, J = 8.5 Hz, 1H), 5.89 (d,

J = 8 Hz, 1H), 3.85 ppm (s, 3H).

5.2.4.6 10-(4-Methoxyphenyl)-10H-phenoxazine-3,7- dicarbaldehyde (6)

6 as an orange solid (0.27g, 46%) was synthesized according to the procedure

described above for the synthesis of 3. To a solution of 2 (0.48g, 0.0017mol)

and dry DMF (1.4mL) in dry1,2-dichloroethane (10mL) in an ice water bath,

POCl3 (9.17mL, 0.1mol)was added dropwise below 15℃. Eluent : DCM.


7.29 (dd, J = 8.5, 1.5 Hz, 2H), 7.24 (d, J = 7 Hz, 2H), 7.16 (s, 2H), 6.02 (d, J = 8

Hz, 2H), 3,86 ppm (s, 3H).

5.2.4.7 (E)-(10-Butyl-10H-phenoxazin-3-yl)-2-cyanoacrylic acid (POX)

3 (0.46g, 0.00173mol), cyanoacetic acid (0.44g, 0.0052mol) and piperidine

(0.35mL, 0.00693mol) were added to anhydrous CH3CN (100mL). After the

mixture was refluxed for 8h, the solution was extracted with DCM and 0.1M

HCl aqueous solution. The organic phase was dried over anhydrous MgSO4 and

the solvent was removed in vacuo. The crude product was purified by column

139

chromatography using DCM- methanol (5:1; v/v) to give POX, red solid (0.43g,

75%). mp 232-233℃.


7.36 (s, 1H), 6.85 (t, J = 7.6 Hz, 1H), 6.69-6.78 (m, 4H), 3.59 (t, J = 7.6 Hz, 2H),

1.50-1.55 (m, 2H), 1.37-1.43 (m, 2H), 0.93 ppm (t, J = 7.3 Hz, 3H) ;

13C NMR ( 150MHz, d6-DMSO) : δ = 164.1, 152.2, 143.8, 143.6, 137.8, 130.9,

130.6, 124.3, 123.7, 122.6, 117.2, 115.3, 114.5, 112.9, 111.7, 98.0, 42.9, 26.7,

19.2, 13.7 ppm ;


ATR-FTIR (cm-1): 2221 (cyano, C≡N stretching band), 1686 (carbonyl, C=O

stretching band).

5.2.4.8 3,3’- (10-Butyl-10H-phenoxazin-3,6-diyl) bis[2-cyanoacrylic acid]

(WB)

WB as a dark red solid (0.56g, 57.3%) was synthesized according to the

procedure described above for the synthesis of POX. 4 (0.67g, 0.0023mol),


added to anhydrous CH3CN (75mL). Eluent : DCM- methanol (2:1; v/v). mp

279-280℃.

140


7.36 (s, 2H), 6.85 (t, J = 7.5 Hz, 2H), 6.69-6.78 (m, 8H), 3.59 (t, J = 7.6 Hz, 2H),

1.50-1.55 (m, 2H), 1.37-1.43 (m, 2H), 0.93 ppm (t, J = 7.3 Hz, 3H) ;

13C NMR (150MHz, d6-DMSO) : δ = 163.7, 152.0, 143.6, 135.6, 130.6, 129.4,

125.3, 116.8, 115.4, 112.9, 43.3, 26.8, 19.2, 13.7 ppm ;



stretching band).

5.2.4.9 (E)-10-((4-Methoxyphenyl)-10H-phenoxazin-3-yl)-2-cyanoacetic acid

(WH1)

WH1 as a red solid (0.37g, 77%) was synthesized according to the procedure

described above for the synthesis of POX. 5 (0.4g, 0.00126mol), cyanoacetic

acid (0.32g, 0.0038mol) and piperidine (0.43mL, 0.00504mol) were added to

anhydrous CH3CN (100mL). Eluent : DCM- methanol (20:1; v/v). mp 266-

267℃.

1H NMR (600MHz, d6-DMSO) : δ = 7.97 (s, 1H), 7.49 (s,1H), 7.36 (d, J = 8.6

Hz, 2H), 7.30 (d, J = 8.6 Hz, 1H), 7.20 (d, J = 8.6 Hz, 2H), 6.79 (d, J = 7.9 Hz,

141

1H), 6.74 (t, J = 7.5 Hz, 1H), 6.68 (t, J = 7.7 Hz, 1H), 5.89-5.91 (m, 2H), 3.84

ppm (s, 3H) ;

13C NMR (150MHz, d6-DMSO) : δ = 163.9, 159.4, 152.4, 143.1, 143.0, 138.9,

132.3, 131.0, 130.6, 129.0, 124.3, 123.9, 122.9, 116.9, 116.5, 115.4, 114.7,

113.9, 112.7, 98.2, 55.4 ppm ;



stretching band).

5.2.4.10 3,3’-10-((4-Methoxyphenyl)-10H-phenoxazin-3,6-diyl) bis[2-

cyanoacrylic acid] (WH2)

WH2 as a dark red solid (0.59g, 53.7%) was synthesized according to the

procedure described above for the synthesis of POX. 6 (0.79g, 0.0023mol),


added to anhydrous CH3CN (75mL). Eluent : DCM-methanol (2:1; v/v). mp

272-273℃.


Hz, 2H), 7.38 (d, J = 8.6 Hz, 2H), 7.24 (d, J = 8.9 Hz, 2H), 6.0 (d, J = 8.5 Hz,

2H), 3.85 ppm (s, 3H).

142

13C NMR (150MHz, d6-DMSO) : δ = 163.5, 159.7, 151.8, 143.1, 137.1, 130.7,

129.6, 128.2, 125.8, 116.7, 115.4, 113.8, 55.5 ppm ;

m/z (FAB) 479.1120 ((M +) , C27H17N3O6 requires 479.1117).


stretching band).


5.3.1. Synthesis of dyes

The detailed synthesis routes to the POZ type dyes are shown in Scheme 5.1.

The POZ dyes were initially modified by N-alkylation or N-arylation of

commercially available POZ. N-alkylation was performed in DMSO with 1-

bromobutane and NaOH as the base, giving intermediate 1. N-arylation was

also performed by the Ullmann reaction, which produced the methoxyphenyl

substituted phenoxazine, intermediate 2. The intermediates 1 and 2 were

subsequently formylated via the Vilsmeyer-Haack reaction to synthesize

intermediates 3-6. During the formylation reactions, the number of introduced

143

aldehyde groups was controlled by varying the amount of POCl3. Finally, the

dyes (POX, WB, WH1 and WH2) were synthesized by Knoevenagel

condensations of the corresponding aldehydes on the intermediate moieties with

cyanoacetic acid [21]. The structures of all the synthesized intermediates and

dyes were confirmed by 1H NMR and the final products were additionally

identified by 13C NMR and HRMS.

Scheme 5.1. Synthesis of POX, WB, WH1 and WH2.

144

5.3.2. Density functional theory (DFT) calculations

To explain the structural properties of the dyes, their geometrically optimized

structures were calculated using the density functional theory (DFT) at the

B3LYP/6-31G(d,p) level. The optimized structures with torsion angle and

electron density of the HOMOs and LUMOs of the dyes are shown in Table 5.1.

In the optimized structures, the POZ moieties of all the dyes exhibited almost

planar structures with small torsion angles (0.06-0.93°). These planar structures

enhanced the aromatic character of the heterocyclic atom, increasing the degree

of electronic resonance between donor and acceptor moieties in the dye

molecules. On the other hand, the planar structure can increase the stacking of

the dye molecules, inducing more dye aggregation. The methoxyphenyl

substituent located on the POZ nitrogen atom made a large dihedral angle of

about 90° with the POZ core. This large dihedral angle caused large steric

hinderance that suppressed aggregations among dye molecules. However, this

gave a small π-system overlap between the substituent and the POZ core, which

limited the HOMO delocalization over the methoxy phenyl substituent. All of

the electron distributions of the HOMO orbitals of the dyes were mostly

localized over the POZ moiety, whereas those of the LUMO orbitals were

145

mainly localized in the cyanoacrylic acid and its adjacent phenyl ring. The

results indicated that the HOMO-LUMO excitation induced by light irradiation

can effectively move the electron distribution from the POZ moiety to the

cyanoacrylic acid moiety. Photoinduced electrons can be efficiently transferred

from the dye to the TiO2 surface by this electron separation.

146

Table 5.1. Optimized structures, dihedral angles and electronic distributions in


Dye Optimized structure HOMO LUMO

POX

WB

WH1

147

WH2

5.3.3. Photophysical properties of the dyes in solution and on TiO2 film

The absorption spectra of the four dyes in THF solution and on the TiO2

surface are shown in Fig. 5.2 and the corresponding photophysical data are

listed in Table 5.2. All the dyes exhibited two major absorption bands at around

310nm and 480nm, respectively. The former band at the shorter wavelength is

due to the localized aromatic π-π* transition, and the latter band at the longer

wavelength is attributed to the intramolecular charge-transfer (ICT) transition

from the donor to the acceptor. The absorption maxima (λmax) of POX, WB,

WH1 and WH2 are 464, 498, 458 and 498 nm, respectively. Di-anchoring dyes

(WB and WH2) were red-shifted in their absorption spectra, compared to the

corresponding mono-anchoring dyes (POX and WH1). This is due to the

extension of electron delocalization over the whole molecule caused by the

introduction of an additional anchoring moiety. WH1 including the N-

148

methoxyphenyl ring showed a slight blue-shift in its absorption spectrum,

compared to POX including the N-butyl chain. As shown in the calculation

study, this is because the N-methoxyphenyl ring, which is perpendicularly to

the POZ plane, gave a poor orbital overlap, which led to inefficient conjugation

[19]. The molar extinction coefficients at λmax of POX, WB, WH1 and WH2

were 38957, 34229, 37224 and 47893 M-1cm-1, respectively. These are higher

than those of conventional TPA and PTZ dyes so the light harvesting of the

dyes and photocurrent generation of the cells are increased. As shown in Fig.

5.2, di-anchoring dyes showed intermediate shoulders in their absorption

spectra at around 400 nm. This might be attributed to the formation of localized

electron transitions in the dye molecules induced by the additional anchoring

group. This phenomenon is also shown in absorption spectra of the dyes on

TiO2 surface.

The absorption spectra of the dyes absorbed on TiO2 films are blue-shifted

compared to those of the dyes in solution. H-aggregation of the dyes or

deprotonation of carboxylic acid upon adsorption onto the TiO2 surfaces has

been suggested as the cause of a blue-shift of the spectrum [22]. The absorption

maxima of the dyes on TiO2 were similarly ranked as those of the dyes in

solution. The dyes with the N-methoxyphenyl ring showed lower absorbance

149

than the dyes with the N-butyl chain. This was ascribed to the lower adsorption

of the dyes with the N-methoxyphenyl ring due to the large steric hinderance.

300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

(a)

POX WB WH1 WH2

Ab

sro

ban

ce (

AU

)

Wavelength (nm)

400 500 6000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

(b)

Ab

sorb

an

ce (

AU

)

Wavelength (nm)

POX WB WH1 WH2

Fig. 5.2. Absorbtion spectra of POX, WB, WH1 and WH2 in (a) THF (10-

5molL-1) and (b) on TiO2.

150

Table 5.2. Photophysical and electrochemical properties of POX, WB, WH1

and WH2.

Dye

Absorptiona Emissiona Oxidation potential datac

λmax/nm

ε/M-1c m-1

(at λmax)

λabsb/nm

(on TiO2)

λmax/nm

Eox/V

(vs. NHE)

E0-0d/V

Eox - E0-0/V

(vs. NHE)

POX 464 38957 444 558 1.01 2.38 -1.37 WB 498 34229 482 565 1.14 2.29 -1.15

WH1 458 37224 426 555 1.06 2.42 -1.36

WH2 498 47893 494 554 1.16 2.33 -1.17

a Measured in 1x 10-5 THF solutions at room temperature.

b Measured on TiO2 film. c Measured in DMF containing 0.1 M tetrabutylammonium tetrafluoroborate. (TBABF4) electrolyte (working electrode: glassy carbon; counter electrode: Pt; reference electrode: Ag/Ag+; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV [23].).

d E0–0 was determined from the intersections of absorption and emission spectra.

5.3.4. Electrochemical properties

The electrochemical properties and energy levels of the synthesized dyes are

provided in Table 5.2 and Fig. 5.3. The oxidation potentials of the dyes (Fig.

5.4) were measured by cyclic voltammetry (CV). The HOMO levels of the four

dyes correspond to the first oxidation potential versus normal hydrogen

151

electrode (vs. NHE) calibrated by Fc/Fc+ (with 640 mV vs. NHE). All the

HOMO levels of the dyes ranged from 1.01 eV to 1.16 eV (vs. NHE) and were

much more positive than the redox potential of I-/I3- (0.4 V vs. NHE) [24].

Therefore, the oxidized dyes were sufficiently regenerated by the redox

electrolyte. Their LUMO levels, corresponding to excited state oxidation

potentials, were calculated by Eox – E0-0, where E0-0 is the zeroth–zeroth energy

of the dye estimated from the intersection between the normalized absorption

and emission spectra (Fig. 5.5 and Fig. 5.6). The LUMO levels of the dyes

ranged from -1.15 to -1.37 and were more negative than the conduction bands

edge of TiO2 (-0.5 vs. NHE) [24], thus indicating that the excited electrons of

the dyes can be efficiently injected into the conduction band of TiO2.

The LUMO levels of the di-anchoring dyes were more positive than those of

mono-anchoring dyes possibly due to the extension of conjugation induced by

the additional anchoring group. Consequently, although the additional

anchoring group made the HOMO level of the di-anchoring group slightly more

positive, the HOMO-LUMO gap decreased and the absorption spectra were

red-shifted.

152

Fig. 5.3. Dyes’ HOMO and LUMO energy levels.

153

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006

0.000008

0.000010

0.000012

0.000014

Cu

rre

nt (

A)

Voltage (V)

Fc POX WB WH1 WH2

Fig.5.4. CV curves of Fc/Fc+, POX, WB, WH1 and WH2 in DMF.

550 600 650 7000

100

200

300

400

500

Em

issi

on In

ten

sity

(A

U)

Wavelength (nm)

POX WB WH1 WH2

Fig. 5.5. Emission spectra of POX, WB, WH1 and WH2 in THF.

154

500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

No

rma

lized

abs

orb

ance

(A

U)

Wavelength (nm)

POX(UV) WB(UV) WH1(UV) WH2(UV) POX(PL) WB(PL) WH1(PL) WH2(PL)

Fig. 5.6. Normalized absorption and emission spectra of POX, WB, WH1 and

WH2 in THF.

5.3.5. Photovoltaic properties

DSSCs were fabricated using the synthesized dyes according to the methods

described in the experimental section and their photovoltaic properties were

measured under standard AM 1.5G irradiation conditions (100mW cm-2). The

incident photon-to-current conversion (IPCE) spectra and photocurrent-voltage

(J-V) curves are shown in Fig. 5.7 and corresponding data are listed in Table

155

5.3. The highest IPCE values of the cells with all dyes were over 79% in the

absorption range ca. 400-600 nm. All the dyes could efficiently convert visible

light to photocurrent in the spectral ranges. The Jsc values of the dyes were

ranked WH1 < WH2 < POX < WB and this trend was in agreement with the

broadness of IPCE spectrum. IPCE spectra of the cells with the di-anchoring

dyes were broader than those with mono-anchoring dyes, promoting higher Jsc

values. This result indicated that the di-anchoring dyes with broad and red-

shifted spectra could inject more electrons effectively into the TiO2 conduction

band. On the other hand, the N-substituted methoxyphenyl ring decreased the

donating ability of the dyes because it had a poor orbital overlap with the POZ

moiety [19]. Therefore, the cells based on the dyes with the N-methoxyphenyl

ring showed narrower IPCEs compared to those with the N-butyl chain,

resulting in lower Jsc values. Photovoltaic properties of DSSCs were affected by

the amount of the dye adsorbed onto the TiO2 surface. As shown in Table 5.3,

the dyes with the N-methoxyphenyl ring were less absorbed than those with the

N-butyl chain because of their bulky structures. As a result, the cells with WH1

and WH2 showed narrower IPCE spectra and lower Jsc values than those with

POX and WB, respectively.

156

Voc values increased in the order of WB < WH2 < POX < WH1. The cells

fabricated with the di-anchoring dyes showed lower Voc values compared to

those with the mono-anchoring dyes. The di-anchoring dyes transfer more

protons to the TiO2 surface upon adsorption [25]. Thus, the high concentration

of the protons changes the TiO2 surface to a more positive state, lowering the

conduction band edges of TiO2 and lowering Voc [26, 27]. To confirm the

binding of the di-anchoring dyes, their FT-IR spectra on the TiO2 surface were

measured (Fig. 5.8). The FT-IR spectra of the di-anchoring dyes showed that

the broad band at 1685 cm-1 assigned to the carboxylic groups of free dyes

disappeared as the dyes were adsorbed on the TiO2, indicating their double-

anchoring behavior [28]. The DSSCs based on the dyes with the N-

methoxyphenyl ring showed higher Voc than those with the N-butyl chain. This

was due to the bulky methoxyphenyl ring that suppressed dye aggregation and

the recombination between the injected electrons on TiO2 and the electrolyte.

Consequently WH1, the mono-anchoring dye with the N-methoxyphenyl ring,

showed the highest Voc, and WB, the di-anchoring dye with the N-butyl chain,

showed the lowest Voc. The dark currents of all DSSCs were also measured (Fig.

5.9); their trends were in good agreement with the Voc ranks of the dyes. Dark

current is defined as the relatively small electric current flowing out of a system

157

without illumination; a higher dark current is accountable for increased

recombination and low Voc. Thus, as shown in Fig. 5.9, the low dark current of

the cell with WH1 is consistent with the highest Voc, whereas the high dark

current of the cell with WB is consistent with the lowest Voc.

All four dyes exhibited almost the same overall conversion efficiencies and

the η values of POX, WB, WH1 and WH2 were 4.97%, 5.03%, 5.09% and

4.86%, respectively. This similarity is ascribed to the trade-off relationship

between Voc and Jsc for the cells based on the synthesized dyes. The best overall

conversion efficiency was achieved by the cell fabricated with WH1 (η = 5.09%,

Jsc = 10.11 mA/cm2, Voc = 690 mV, FF = 72.23%).

158

300 400 500 600 7000

20

40

60

80

100

(a)

IPC

E (

%)

Wavelength (nm)

POX WB WH1 WH2

0.0 0.2 0.4 0.6 0.80

2

4

6

8

10

12 (b)

Pho

tocu

rren

turr

ent d

ensi

ty (

mA

/cm

2)

Voltage (V)

POX WB WH1 WH2

Fig. 5.7. (a) IPCE spectra DSSCs based on POX, WB, WH1 and WH2 and (b)

the DSSCs' J–V curves under AM 1.5G simulated sunlight (100 mWcm-2).

159

Table 5.3. DSSC performance parameters of POX, WB, WH1, and WH2.a

Dyeb Jsc

(mA/cm2)

Voc

(V)

FF

(%)

η

(%)

Dye amountc

(10–7mol cm–2)

POX 11.19 0.68 64.97 4.97 1.4207

WB 11.79 0.65 65.10 5.02 1.8653

WH1 10.11 0.69 72.23 5.09 1.2744

WH2 10.75 0.66 67.5 4.86 1.2370

N719 14.89 0.78 63.18 7.69 -

a Measured under AM 1.5 irradiation G (100 mW cm–1); 0.236~0.3 cm2 working area. b Dyes

were maintained at 0.5 mM in THF solution, with 10 mM CDCA co-adsorbent. Electrolyte

comprised 0.7 M 1-propryl-3-methyl-imidazolium iodide (PMII), 0.2 M LiI, 0.05 M I2, 0.5 M

TBP in acetonitrile-valeronitrile (v/v, 85/15) for organic dyes. c Dyes' adsorption on TiO2 were

measured by a colorimetric method using 0.1 M NaOH aqueous-DMF (1:1) mixed solutions to

wash the dyes from 8 μm thick TiO2 film.

160

1000 1500 2000 2500 3000 3500 400070

75

80

85

90

95

100(a)

Tra

nsm

itta

nce

(%)

Wavelength (cm-1)

WB

1000 1500 2000 2500 3000 3500 400080

82

84

86

88

90

92

94

96

98

100

102(b)

Tra

nsm

itta

nce

(%)

Wavelength (cm-1)

WH2

Fig. 5.8. FT-IR spectra of (a) WB absorbed on TiO2 and (b) WH2 absorbed on

TiO2.

161

0.0 0.2 0.4 0.6 0.8

-4

-2

0

2

C

urr

en

t (m

A/c

m2)

Voltage (V)

POX WB WH1 WH2

Fig. 5.9. DSSCs' J–V curves based on POX, WB, WH1 and WH2 in the dark.

5.3.6. Electrochemical impedance spectroscopy

To elucidate the correlation between the Voc of the cell and the dye,

electrochemical impedance spectroscopy (EIS) [29] was carried out in the dark

and under illumination. A Nyquist plot in the dark under a forward bias of

0.55V with frequency range of 0.1Hz-100kHz and a Bode phase plot under AM

1.5GmWcm-2 illumination are shown in Fig. 5.10.

162

The major semicircle in the Nyquist plot indicate charge recombination

resistance at the TiO2 surface; a larger radius of the major semicircle means a

larger charge recombination resistance [30]. The radius of the major semicircle

in the Nyquist plot increased in the order of WB < WH2 < POX < WH1,

implying increasing resistance to charge recombination. This result coincides

with the increase of Voc in the DSSCs based on the dyes, as the suppresion of

electron recombination between the injected electrons and electrolyte improves

Voc. This trend is in agreement with results of the electron lifetime vs dark bias

voltage (Fig. 5.11). Electron lifetime is calculated by multiplying the resistance

by the chemical capacitance (Fig. 5.12 and Table 5.4). The chemical

capacitances of the dyes were almost similar, so that electron lifetimes were

mainly dependent on the resistances. Thus, longer electron lifetimes imply

increased resistance between the injected electrons and the electrolyte, which

consequently improves the Voc [31].

A Bode phase plot is also related to the charge transfer resistance at the

TiO2/dye/electrolyte interface. The frequency of the characteristic peak in the

Bode phase plot increased in the order of WH1 < POX < WH2 < WB. A lower

characteristic frequency in the Bode phase plot means a slower charge

recombination rate and higher Voc. As the reciprocal of the characteristic

163

frequency is correlated with electron lifetime, electron lifetime increased in the

order of WB < WH2 < POX < WH1 [32]. This sequence of the electron

lifetime calculated from the Bode phase plot also indicated increasing resistance

to recombination. These results suggest that the introduction of the methoxy

phenyl ring improved Voc, whereas the introduction of the additional anchoring

group decreased Voc.

0 50 100 150 200 250 3000

-50

-100

-150

-200

-250

-300

(a)

Z"

( )

Z'()

POX WB WH1 WH2

10-1 100 101 102 103 104 105

0

-2

-4

-6

-8

-10

(b)

Z"

( )

Frequency (Hz)

POX WB WH1 WH2

Fig. 5.10. Impedance spectra of DSSCs based on POX, WB, WH1, and WH2.

(a) Nyquist plots measured at 0.55 V forward bias in the dark, (b) Bode phase

plots measured under illuminations (AM 1.5G).

164

0.3 0.4 0.5 0.6 0.710

100

1000

10000

POX WB WH1 WH2

Life

time

(ms)

Bias voltage(V)

Fig. 5.11. Electron lifetime of POX, WB, WH1, and WH2 as a function of bias

voltage.

165

0.2 0.3 0.4 0.5 0.6 0.7 0.8

10

100

1000

10000

(a) POX WB WH1 WH2

Res

ista

nce

(c

m2 )

Bias voltage (V)

0.3 0.4 0.5 0.6 0.71E-4

1E-3

0.01

(b)

POX WB WH1 WH2

Ca

pac

itan

ce (

F*c

m2)

Bias voltage (V)

Fig. 5.12. (a) Resistance and (b) capacitance of POX, WB, WH1 and WH2 as

a function of bias voltage.

166

Table 5.4. Lifetime calculations of POX, WB, WH1 and WH2.

Device Rsa R2a C2a

R2*C2

Life time

(ms)

POX 4.94 18.44 6.16*E-04 11.35

WB 4.99 13.26 5.22*E-04 6.92

WH1 4.58 15.07 8.42*E-04 12.68

WH2 4.98 15.29 6.93*E-04 10.60

a Rs, R2 (Rrec) and C2 are the ohmic serial resistance, charge transfer resistance at

TiO2/dye/electrolyte interface and chemical capacitance, respectively.

167

5.4 Conclusion

Four new organic sensitizers with POZ dyes (POX, WB, WH1 and WH2)

were designed and synthesized to investigate the effects of an additional

anchoring group and the N-substituent on the performance of DSSCs.

The introduction of the additional anchoring group extended the conjugation

of the dyes, leading to the red-shifts of their absorption spectra. The di-

anchoring dyes transferred more electrons to the TiO2 electrode, increasing the

Jsc value. The introduction of the methoxyphenyl ring located on the POZ

nitrogen atom caused steric hinderance due to the large dihedral angle between

the N-substituent and the POZ core. As a result, the electron recombination

between the injected electron and the electrolyte was retarded, improving the

Voc value.

On the other hand, the di-anchoring dyes transferred more protons to the

TiO2 surface compared to the mono-anchoring dyes. This lowered the Fermi

level of the TiO2 and the Voc of the cells. The adsorption of dyes with the N-

methoxyohenyl ring was limited by their bulky structures. Therefore, the dyes

168

with the N-methoxyohenyl ring showed lower Jsc values compared to the dyes

with the butyl chain.

Consequently, introduction of the additional anchoring group enhanced Jsc,

but decreased Voc. To improve the lowered Voc caused by the di-anchoring

system, a methoyphenyl ring was introduced on the POZ core. The introduction

of the 4-methoxyphenyl ring increased the Voc, but decreased the Jsc

simultaneously. Thus, with the increases and decreases of Jsc and Voc, all

DSSCs fabricated with the dyes showed similar overall conversion efficiencies,

and the cells based on WH1 showed the best overall conversion efficiency (η =

5.09%, Jsc = 10.11 mA/cm2, Voc = 690 mV, FF = 72.23%).

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177

Summary

Three solubility enhanced phthalocyanine dyes were synthesized, and the dye-

based BMs were fabricated with the most soluble dye. The increase in solubility

of the prepared dyes was attributed to bulky functional substituents at the

peripheral positions of them. Since all dyes had high molar extinction

coefficients, dye-based BMs absorbed light in the visible region with the small

amounts of the dyes. In addition, the dyes including terminal alkoxy groups

showed suitable thermal stability for commercial use due to terminal alkoxy

groups are stable at postbaking temperature. The dielectric constants of the

BMs containing more than 30wt% of dyes were significantly lower than that of

the BM prepared with carbon black only. The dye-based BM films were

fabricated with greenish phthalocyanine and reddish perylene dyes. The high

thermal stability of the dye-based BM was attributed to the rigid molecular

structures of the dyes. In addition, due to the low dielectric characteristics of the

dye, the dielectric constants of the dye-based BMs were significantly lower than

that of the BM prepared with carbon black only. However, the low solubility of

the dyes in industrial solvents and dye aggregations in the baking process

limited the input of the dye in the BM resist, resulting in low light absorption of

the dye-based BM. By fabricating hybrid-type BM that includes dye and carbon

178

black together, the light absorption property of the BM would be improved

compared to the dye-based BM, satisfying the property requirements of BMs.

The first design and synthesis of three novel phenoxazine-based organic dyes

(WS1, WS2 and WS3) has been done to study the effects of the various bridge

units and an additional donor on the performance of DSSCs. The introduction

of the heterocyclic bridge units (furan and thiophene) extended the conjugation

and red-shifted the absorption spectra of the dyes, improving Jsc. Consequently,

the DSSCs based on WS1 with the furan unit and WS2 with the thiophene unit

showed higher overall conversion efficiencies in comparison with the reference

dye (POX) without the bridge unit. The most effective bridge unit was furan,

which had a higher recombination resistance and Voc value. WS3 with the

ethoxy phenyl ring as an additional donor showed more red-shift in the

absorption, whereas the planarity of the dye was reduced by the large dihedral

angle between the ethoxy phenyl ring and the POZ core. This non-planar

structure of the dye led to less adsorption of the dye on the TiO2 surface, which

limited the Jsc enhancement. The DSSC based on WS1 with the furan bridge

unit exhibited the highest overall conversion efficiency of 5.26% (Jsc = 11.72

mA/cm2, Voc = 653 mV, FF = 68.76). In addition, four new organic sensitizers

with POZ dyes (POX, WB, WH1 and WH2) were designed and synthesized to

179

investigate the effects of an additional anchoring group and the N-substituent on

the performance of DSSCs. The introduction of the additional anchoring group

extended the conjugation of the dyes, leading to the red-shifts of their

absorption spectra. The di-anchoring dyes transferred more electrons to the

TiO2 electrode, increasing the Jsc value. The introduction of the methoxyphenyl

ring located on the POZ nitrogen atom caused steric hinderance due to the large

dihedral angle between the N-substituent and the POZ core. As a result, the

electron recombination between the injected electron and the electrolyte was

retarded, improving the Voc value. On the other hand, the di-anchoring dyes

transferred more protons to the TiO2 surface compared to the mono-anchoring

dyes. This lowered the Fermi level of the TiO2 and the Voc of the cells. The

adsorption of dyes with the N-methoxyohenyl ring was limited by their bulky

structures. Therefore, the dyes with the N-methoxyohenyl ring showed lower Jsc

values compared to the dyes with the butyl chain. Consequently, introduction of

the additional anchoring group enhanced Jsc, but decreased Voc. To improve the

lowered Voc caused by the di-anchoring system, a methoyphenyl ring was

introduced on the POZ core. The introduction of the 4-methoxyphenyl ring

increased the Voc, but decreased the Jsc simultaneously. Thus, with the increases

and decreases of Jsc and Voc, all DSSCs fabricated with the dyes showed similar

180

overall conversion efficiencies, and the cells based on WH1 showed the best

overall conversion efficiency (η = 5.09%, Jsc = 10.11 mA/cm2, Voc = 690 mV,

FF = 72.23%).

181

초록

액정디스플레이 (LCD) 광차단막으로 가장 빈번하게 쓰이는 물질은 카본 블

랙으로, 높은 내열성과 높은 흡광도를 가진다. 반면, 카본블랙은 높은 유전

율을 가지기 때문에 광차단막이 박막트랜지스터 바로 위에 형성되는 구조를

가지는 LCD에서는 오작동을 일으킬 가능성이 있다. 이러한 문제를 해결하

기 위해서 광차단막 재료로 저유전율을 가지는 유기 안료를 사용할 수 있으

나, 유기안료는 몰흡광도가 낮기 때문에 상대적으로 낮은 광학 특성을 가진

다.

유기염료는 낮은 유전율을 가지는 동시에 높은 몰흡광도를 보이기 때문에

위의 두 재료가 가진 단점을 극복할 수 있는 광차단막 물질이 될 수 있다.

반면, 유기염료는 일반적으로 카본블랙이나 유기안료에 비해서 내열성이 낮

은 단점이 있으며, 유기염료가 광차단막 제작 공정에 적용되기 위해서는 공

정 용매에 높은 용해도를 가져야 한다. 따라서, 높은 내열성을 가지며, 동시

에 공정용매에 대해서 높은 용해도를 보이는 유기 염료를 개발이 필요하다.

본 연구에서는 고내열성 및 고용해도를 가지며 녹색을 띠는 금속 없는 프

탈로시아닌 염료 3종을 설계하고 합성하였다. 세 염료의 합성은 알킬기 또

는 알콕시기를 포함하는 치환체를 프탈로시아닌 염료의 peripheral 위치에

182

도입을 통해 이루어졌다. 광차단막에 적용 가능성을 알아보기 위해서 합성

된 염료의 광학적 특성, 용해도, 내열성을 측정하였으며, 염료로 제작한 광

차단막의 광학적, 열적, 유전적 특성을 조사하였다.

금속 없는 프탈로시아닌 염료의 용해도는 염료에 peripheral 위치에 도입

된 부피가 큰 치환체로 인해서 상승하였다. 또한 모든 염료는 높은 몰흡광

계수를 가지며, 염료를 이용하여 제작한 광차단막도 가시광선 영역에서 넓

은 범위의 광 흡수를 보였다. 따라서 상대적으로 적은 양의 염료로도 높은

광 차단 특성을 나타낼 수 있는 가능성을 보여주었다. 염료들 중에서 알콕

시기를 포함하는 염료의 경우 염료 광차단막 공정에 적용 가능한 우수한 내

열성을 보였다. 또, 제작된 염료기반 광차단막의 유전율은 카본블랙으로 제

작된 광차단막에 비해서 상당히 낮은 수치를 보였다. 따라서 카본블랙과 염

료를 혼합하여 만든 광차단막 중에서 30wt%이상의 염료를 함유하는 경우

저유전율 광차단막에 적용할 수 있음을 확인 하였다.

한편, 앞서 합성한 금속 없는 프탈로시아닌 염료는 가시광선 영역에서

550nm 근처영역의 빛을 흡수하지 못한다. 그러므로 가시광선 점 범위를 흡

수하는 광차단막을 제작하기 위해서, 녹색을 띠는 zinc 프탈로시아닌 염료와

적색을 나타내는 퍼릴렌 염료를 합성하고 두 염료를 혼합하여 광차단막을

제작하였다. 합성한 두 염료 자체의 특성을 조사하였으며, 두 염료를 혼합하

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여 제작한 광차단막의 광학적, 열적, 유전적 특성을 측정하였다. 또 제작한

광차단막의 표면 상태를 자세히 보기 위해서 FE-SEM과 AFM을 이용하여

광차단막의 표면을 조사하였다.

Zinc 프탈로시아닌 염료와 퍼릴렌 염료를 혼합하여 만든 광차단막은 높은

내열성을 나타냈으며, 이는 염료 모체 구조의 열적 안정성에 기인하였다. 또,

유전율이 낮은 염료의 특성으로 인해서, 광차단막 역시 카본 블랙 기반 광

차단막에 비해서 낮은 유전율을 나타냈다. 반면, 프탈로시아닌 염료의 공정

용매에 대한 낮은 용해도와 열처리 과정에서 형성된 염료 회합 현상으로 인

해 광차단막의 흡광 특성이 낮아지게 되었다. 본 연구로부터 유기 염료가

광차단막에 더 성공적으로 적용되기 위해서는 분산제 혹은 계면활성제의 첨

가를 통해서 염료의 투입량을 증가시켜야 하며, 더불어서 염료 단독 보다

염료와 카본 블랙을 혼합하여 만드는 것이 필요하다는 것을 확인하였다.

염료감응형 태양전지는 전도유망한 태양전지 중 하나로 그 동안 많은 관

심을 받아왔으며, 염료감응형 태양전지의 광감응제로 가장 활발히 연구되었

던 루세늄을 포함한 염료를 통해서 11%에 달하는 높은 광전변환효율을 달

성하였다. 반면, 높은 생산 단가, 정제의 어려움 같은 단점들이 있어서 상업

적인 적용에 어려움이 있었다. 최근에는 이러한 단점을 극복하기 위해서 루

세늄을 포함하지 않는 유기 염료에 대해서 많은 관심이 집중되고 있으며,

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이러한 유기 염료는 낮은 생산 단가, 구조 변환 및 합성의 용이성, 높은 몰

흡광계수, 친환경성 같은 장점을 가지고 있다.

루세늄을 포함하지 않은 여러 유기 염료들 중에서 페녹사진 염료는 비슷

한 구조인 트리페닐아민, 페노사이아진 염료에 비해서 상대적으로 높은 광

전변환효율을 나타내는 것으로 알려져 있다. 이는 전자가 풍부한 질소, 산소

원자를 포함하고 있는 페녹사진 염료가 더 강한 전자 공여 능력을 가지는데

기인한다. 또한 페녹사진 염료는 염료감응형 태양전지에 도입되기 충분한

전기화학적 특성을 가지고 있다. 이러한 장점에도 불구하고, 염료감응형 태

양전지용 광감응제로서 페녹사진 염료에 대한 많은 연구가 이루어지지 않았

다.

따라서, 본 연구에서는 페녹사진 염료에 conjugated bridge 도입을 통한

효과를 연구하기 위해서 페녹사진 모체에 conjugated bridge역할을 하는

five-membered heterocyclic rings 을 도입하였다. 또, 전자 공여 능력과 몰

흡광도를 향상시키기 위해서 furan이 달린 페녹사진 염료에 추가적인 전자

주개 그룹인 에톡시 페닐링을 달아주었다. 이러한 전략을 통해서 세 종의

페녹사진 염료를 설계하고 합성하였으며, 도입된 치환체의 효과를 설명하기

위해서 합성한 염료들로 만든 셀의 광학적, 전기화학적 특성 및 광전기적

특성을 측정하였다.

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도입된 heterocyclic bridge units 인 퓨란과 싸이오펜은 염료의 흡광 스펙트

럼의 장파장화를 시켜서 염료를 이용하여 만든 셀의 광전밀도를 향상시켰다.

추가적인 전자 주개 그룹인 에톡시 페닐링의 도입은 염료의 흡광 스펙트럼

을 장파장화시켰으나, 염료 분자 구조를 뒤틀리게 하여 흡착량을 감소시켰

다. 따라서 에톡시 페닐링이 도입된 염료의 광전밀도는 크게 향상 되지 못

했다. 합성된 염료들 중에서는 퓨란이 달린 염료로 만든 셀이 가장 높은 광

전 변환효율을 나타냈으며, 5.26%의 수치를 나타내었다.

더불어, 페녹사진의 염료에 추가적인 전자 받개 그룹을 도입하여 그에 따

른 효과를 알아보고자 하였다. 또 전자 받개 그룹이 두 개 달린 염료의 개

방전압을 향상시키기 위해서 페녹사진 염료의 질소가 있는 위치에 부피가

큰 메톡시 페닐링을 도입하였다. 이와 같은 계획을 통해서 총 네 종의 염료

를 설계하고 합성하였으며, 도입된 치환체의 효과를 설명하기 위해서 염료

들을 이용하여 만든 셀의 광학적, 전기화학적 특성 및 광전기적 특성을 측

정하였다.

추가적인 전자 주개 그룹을 도입함으로 인해서 흡광 스펙트럼이 장파장화

하여 전류밀도가 향상되었으나, 전자재결합 속도가 높아지는 단점이 있었다.

도입된 N-메톡시 페닐링은 전자재결합을 방해하여 개방 전압 값을 향상시

켰다. 반면, 부피가 큰 N-메톡시 페닐링으로 인하여 염료의 흡착량이 줄어

186

들었다. 결과적으로 전류밀도 및 개방 전압값의 증감에 따라서 네 염료 모

두 거의 비슷한 효율을 나타내었으며, 합성한 염료 중에서 N-메톡시 페닐링

이 달린 염료로 제작한 셀이 가장 높은 광전 변환효율인 (5.09%)을 나타내

었다.

187

List of Publications

Original Papers

1. W. Lee, S. B. Yuk, J. Choi, D. H. Jung, S. Choi, J. Park, J. P. Kim “Synthesis

and characterization of solubility enhanced metal-free phthalocyanines for

liquid crystal display black matrix of low dielectric constant”, Dyes and

Pigments, 2012, 92, 942 (article)

2. J. Choi, W. Lee, C. Sakong, S. B. Yuk, J. Park, J. P. Kim “Facile synthesis

and characterization of novel coronene chromophores and their application to

LCD color filters”, Dyes and Pigments, 2012, 94, 34 (article)

3. J. Choi, S. H. Kim, W. Lee, C. Yoon, J. P. Kim “Synthesis and

characterization of thermally stable dyes with improved optical properties for

dye-based LCD color filters”, New J. Chem, 2012, 36, 812 (article)

4. J. Choi, W. Lee, J. W. Namgoong, T. Kim, J. P. Kim “Synthesis and

characterization of novel triazatetrabenzcorrole dyes for LCD color filter and

black matrix”, Dyes and Pigments, 2013, 99, 357 (article)

5. J. Lee, S. H. Kim, W. Lee, J. Lee, B. An, S. Y. Oh, J. P. Kim, J. Park

“ Electrochemical and optical characterization of cobalt, copper and zinc

phthalocyanine complex”, Journal of Nanoscience and Nanotechnology, 2013,

188

13, 4338 (article)

6. W. Lee, J. Choi, S. H. Kim, J. Park, J. P. Kim “Analysis and characterization

of dye-based black matrix film of low dielectric constant containing

phthalocyanine and perylene dyes”, Journal of Nanoscience and

Nanotechnology, accepted

7. W. Lee, S. B. Yuk, J. Choi, H. J. Kim, H. W. Kim, S. H. Kim, B. Kim, M. J.

Ko, J. P. Kim “The effects of the number of anchoring groups and N-

substitution on the performance of phenoxazine dyes in dye-sensitized solar

cells” , Dyes and Pigments, 2014, 102, 13-21

8. W. Lee, J. Choi, J. W. Namgoong, S. H. Kim, K. C. Sun, S. H. Jung, K. Yoo,

M. J. Ko, J. P. Kim “Effects of five-membered heterocyclic bridges and

ethoxyphenyl substitution on the performance of phenoxazine-based dye-

sensitized solar cells” accepted

9. J. Choi, S. H. Kim, W. Lee, J. B. Chang, C. Sakong, J. W. Namgoong, J. P.

Kim

“The Influence of aggregation behavior of novel quinophthalone dyes on

optical and thermal property of LCD color filters”, Dyes and Pigments, 2014,

101, 186-195

189

List of Presentations

International

1. 2008 Korea-Japan Forum, W. Lee, J. P. Kim “Synthesis and characterization

of novel color compensating dyes for PDP”

2. 2010 International Conference on Porphyrins and Phthalocyanines, W. Lee, S.

B. Yuk, J. H. Choi, J. P. Kim “Synthesis and characterization of highly soluble

metal-free phthalocyanines for LCD black matrix”

3. 2012 19th International Conference on Photochemical Conversion and

Storage of Solar Energy, W. Lee, J. P. Kim “Modification of phenoxazine dyes

for efficient sensitizers in dye-sensitized solar cells”

4. 2012 19th International Conference on Photochemical Conversion and

Storage of Solar Energy, S. B. Yuk, W. Lee, J. W. Namgoong, J. P. Kim “The

effect of additional electron donating group and conjugated linker on the

efficiency of DSSCs based on phenoxazine dyes”

190

Domestic

1. 2009 춘계 대한화학회, 김현우, 사공천, 이우성, 김재필 “Synthesis

and photovoltaic properties of organic dyes containing different hetero atoms in

donor group”

2. 2009 추계 대한화학회, 이우성, 육심범, 김재필 “Syntheses and

Properties of Novel Metal-free Phthalocyanines derived from sterically

hindered Phenols”

3. 2010 추계 대한화학회, 이우성, 김재필 “Synthesis and Photovoltaic

properties of efficient oxazine dyes for DSSCs”

4. 2011 춘계 대한화학회, 이우성, 김재필 “Phenoxazine derivatives with

heterocyclic five-membered bridge unit for efficient sensitizers in dye-

sensitized solar cells”

5. 2011 춘계 대한화학회, 최준, 이우성, 김재필 “Synthesis and

characterization of novel coronene chromophores”

6. 2011 춘계 대한화학회, 육심범, 이우성, 김재필 “Synthesis of bay-

substituted perylene diimide dyes for black matrix of liquid crystal display”

synthesis and characterization of...

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